[cling] Delete globals before JIT deletes the module:

Fixes root-meta-execUnloading which shows
```==768591== Invalid read of size 8
==768591==    at 0x6EFB10E: llvm::ilist_node_base<false>::getNext() const (ilist_node_base.h:29)
==768591==    by 0x6FBF119: llvm::ilist_node_impl<llvm::ilist_detail::node_options<llvm::Function, false, false, void> >::getNext() (ilist_node.h:66)
==768591==    by 0x6FBED48: llvm::ilist_iterator<llvm::ilist_detail::node_options<llvm::Function, false, false, void>, false, false>::operator++() (ilist_iterator.h:157)
==768591==    by 0x6FBEA20: llvm::simple_ilist<llvm::Function>::begin() (simple_ilist.h:117)
==768591==    by 0x6FBE40F: llvm::Module::begin() (Module.h:622)
==768591==    by 0x6FBE475: llvm::Module::functions() (Module.h:634)
==768591==    by 0x6FBE22F: cling::TransactionUnloader::unloadModule(llvm::Module*) (TransactionUnloader.cpp:175)
==768591==    by 0x6FBDF83: cling::TransactionUnloader::RevertTransaction(cling::Transaction*) (TransactionUnloader.cpp:126)
==768591==    by 0x6F90CD0: cling::Interpreter::unload(cling::Transaction&) (Interpreter.cpp:1553)
==768591==    by 0x6F90E25: cling::Interpreter::unload(unsigned int) (Interpreter.cpp:1574)
==768591==    by 0x721A9F2: cling::MetaSema::actOnUndoCommand(unsigned int) (MetaSema.cpp:186)
```
...
```
==768591==  Address 0x144b0ce0 is 32 bytes inside a block of size 800 free'd
==768591==    at 0x484399B: operator delete(void*, unsigned long) (vg_replace_malloc.c:935)
==768591==    by 0x6FBC7CA: std::default_delete<llvm::Module>::operator()(llvm::Module*) const (unique_ptr.h:95)
==768591==    by 0x6FBC82D: std::__uniq_ptr_impl<llvm::Module, std::default_delete<llvm::Module> >::reset(llvm::Module*) (unique_ptr.h:203)
==768591==    by 0x6FBC87E: std::unique_ptr<llvm::Module, std::default_delete<llvm::Module> >::reset(llvm::Module*) (unique_ptr.h:501)
==768591==    by 0x6FBC04E: std::unique_ptr<llvm::Module, std::default_delete<llvm::Module> >::operator=(decltype(nullptr)) (unique_ptr.h:434)
==768591==    by 0x70E60E7: llvm::orc::ThreadSafeModule::~ThreadSafeModule() (ThreadSafeModule.h:116)
==768591==    by 0x936EC79: llvm::orc::IRMaterializationUnit::~IRMaterializationUnit() (Layer.h:31)
==768591==    by 0x93707F5: llvm::orc::BasicIRLayerMaterializationUnit::~BasicIRLayerMaterializationUnit() (Layer.h:121)
==768591==    by 0x9370811: llvm::orc::BasicIRLayerMaterializationUnit::~BasicIRLayerMaterializationUnit() (Layer.h:121)
==768591==    by 0x6E91B45: std::default_delete<llvm::orc::MaterializationUnit>::operator()(llvm::orc::MaterializationUnit*) const (unique_ptr.h:95)
==768591==    by 0x6E8F9A3: std::unique_ptr<llvm::orc::MaterializationUnit, std::default_delete<llvm::orc::MaterializationUnit> >::~unique_ptr() (unique_ptr.h:396)
==768591==    by 0x935D3C3: llvm::orc::JITDylib::UnmaterializedInfo::~UnmaterializedInfo() (Core.h:1062)
==768591==    by 0x935D3DE: void std::_Destroy<llvm::orc::JITDylib::UnmaterializedInfo>(llvm::orc::JITDylib::UnmaterializedInfo*) (stl_construct.h:151)
==768591==    by 0x935D26B: void std::allocator_traits<std::allocator<void> >::destroy<llvm::orc::JITDylib::UnmaterializedInfo>(std::allocator<void>&, llvm::orc::JITDylib::UnmaterializedInfo*) (alloc_traits.h:648)
==768591==    by 0x935CBA0: std::_Sp_counted_ptr_inplace<llvm::orc::JITDylib::UnmaterializedInfo, std::allocator<void>, (__gnu_cxx::_Lock_policy)2>::_M_dispose() (shared_ptr_base.h:613)
==768591==    by 0x6C39FCE: std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release() (shared_ptr_base.h:346)
==768591==    by 0x6C3CB36: std::__shared_count<(__gnu_cxx::_Lock_policy)2>::~__shared_count() (shared_ptr_base.h:1071)
==768591==    by 0x6E98CED: std::__shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo, (__gnu_cxx::_Lock_policy)2>::~__shared_ptr() (shared_ptr_base.h:1524)
==768591==    by 0x6E98D09: std::shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo>::~shared_ptr() (shared_ptr.h:175)
==768591==    by 0x932D614: llvm::DenseMapBase<llvm::DenseMap<llvm::orc::SymbolStringPtr, std::shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo>, llvm::DenseMapInfo<llvm::orc::SymbolStringPtr>, llvm::detail::DenseMapPair<llvm::orc::SymbolStringPtr, std::shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo> > >, llvm::orc::SymbolStringPtr, std::shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo>, llvm::DenseMapInfo<llvm::orc::SymbolStringPtr>, llvm::detail::DenseMapPair<llvm::orc::SymbolStringPtr, std::shared_ptr<llvm::orc::JITDylib::UnmaterializedInfo> > >::erase(llvm::orc::SymbolStringPtr const&) (DenseMap.h:307)
==768591==    by 0x931028D: llvm::orc::JITDylib::removeTracker(llvm::orc::ResourceTracker&) (Core.cpp:1510)
==768591==    by 0x9315060: llvm::orc::ExecutionSession::removeResourceTracker(llvm::orc::ResourceTracker&)::{lambda()#1}::operator()() const (Core.cpp:2155)
==768591==    by 0x93150D0: decltype(auto) llvm::orc::ExecutionSession::runSessionLocked<llvm::orc::ExecutionSession::removeResourceTracker(llvm::orc::ResourceTracker&)::{lambda()#1}>(llvm::orc::ExecutionSession::removeResourceTracker(llvm::orc::ResourceTracker&)::{lambda()#1}&&) (Core.h:1326)
==768591==    by 0x9315272: llvm::orc::ExecutionSession::removeResourceTracker(llvm::orc::ResourceTracker&) (Core.cpp:2152)
==768591==    by 0x93060FD: llvm::orc::ResourceTracker::remove() (Core.cpp:53)
==768591==    by 0x70F1D58: cling::IncrementalJIT::removeModule(cling::Transaction const&) (IncrementalJIT.cpp:151)
==768591==    by 0x6FBE6CF: cling::IncrementalExecutor::unloadModule(cling::Transaction const&) const (IncrementalExecutor.h:180)
==768591==    by 0x6FBDE5B: cling::TransactionUnloader::RevertTransaction(cling::Transaction*) (TransactionUnloader.cpp:119)
==768591==    by 0x6F90CD0: cling::Interpreter::unload(cling::Transaction&) (Interpreter.cpp:1553)
==768591==    by 0x6F90E25: cling::Interpreter::unload(unsigned int) (Interpreter.cpp:1574)
==768591==    by 0x721A9F2: cling::MetaSema::actOnUndoCommand(unsigned int) (MetaSema.cpp:186)
```

interpreter/cling/lib/Interpreter/TransactionUnloader.cpp

@@ -116,11 +116,6 @@ namespace cling {
 
     bool Successful = true;
     if (getExecutor()) {
-      if (llvm::Error Err = getExecutor()->unloadModule(*T)) {
-        llvm::logAllUnhandledErrors(std::move(Err), llvm::errs(), "Unload: ");
-        Successful = false;
-      }
-
       if (T->getState() == Transaction::kCommitted && !T->isNestedTransaction()) {
         if (const llvm::Module *CompiledM = T->getCompiledModule())
           Successful = unloadModule(const_cast<llvm::Module*>(CompiledM)) &&
@@ -130,6 +125,13 @@ namespace cling {
                  && "Must have already compiled this module");
       }
 
+      // getExecutor()->unloadModule() will free globals - so first run
+      // this->unloadModule().
+      if (llvm::Error Err = getExecutor()->unloadModule(*T)) {
+        llvm::logAllUnhandledErrors(std::move(Err), llvm::errs(), "Unload: ");
+        Successful = false;
+      }
+
       // Cleanup the module from unused global values.
       // if (T->getModule()) {
       //   llvm::ModulePass* globalDCE = llvm::createGlobalDCEPass();