aotcompile: destroy LLVM context after serializing combined module (#59329)

We already save some memory here by deleting the `jl_native_code_desc_t`
after
we're done serializing the combined module, but some data in the
module's
`LLVM::Context` lives on until the end of the scope in
`jl_dump_native_impl`.
Once we're done with the module, move the lock and `ThreadSafeContext`
so the
reference count drops to zero.

A crude measurement shows that when compiling the Base sysimage, about 3
GiB is
in use. Deleting the `jl_native_code_desc_t` (as before) saves about 600
MiB,
and cleaning up the context saves an additional ~500 MiB.

(cherry picked from commit ceeb661)

Author: xal-0

src/aotcompile.cpp

@@ -2169,9 +2169,14 @@ void jl_dump_native_impl(void *native_code,
         auto lock = TSCtx.getLock();
         auto dataM = data->M.getModuleUnlocked();
 
-        // Delete data when add_output thinks it's done with it
-        // Saves memory for use when multithreading
-        data_outputs = compile(*dataM, "text", threads, [data](Module &) { delete data; });
+        data_outputs = compile(*dataM, "text", threads, [data, &lock, &TSCtx](Module &) {
+            // Delete data when add_output thinks it's done with it
+            // Saves memory for use when multithreading
+            auto lock2 = std::move(lock);
+            delete data;
+            // Drop last reference to shared LLVM::Context
+            auto TSCtx2 = std::move(TSCtx);
+        });
     }
 
     if (params->emit_metadata) {