Work around a module build failure on the bots.

This patch works around what looks like a bug in Clang itself.

The error on the bot is:

https://green.lab.llvm.org/green/view/LLDB/job/lldb-cmake/40466/consoleText

In module 'LLVM_Utils' imported from /Users/buildslave/jenkins/workspace/lldb-cmake/llvm-project/lldb/source/Plugins/ScriptInterpreter/Python/lldb-python.h:18:
/Users/buildslave/jenkins/workspace/lldb-cmake/llvm-project/llvm/include/llvm/Support/Error.h:720:3: error: 'llvm::Expected<bool>::(anonymous)' from module 'LLVM_Utils.Support.Error' is not present in definition of 'llvm::Expected<bool>' in module 'LLVM_Utils.Support.Error'
  union {
  ^
/Users/buildslave/jenkins/workspace/lldb-cmake/llvm-project/llvm/include/llvm/Support/Error.h:720:3: note: declaration of '' does not match
/Users/buildslave/jenkins/workspace/lldb-cmake/llvm-project/llvm/include/llvm/Support/Error.h:720:3: note: declaration of '' does not match
1 error generated.

The intention is to revert this as soon as a proper fix has been identified!

rdar://87845391

Author: adrian-prantl

lldb/source/Plugins/ScriptInterpreter/Python/lldb-python.h

@@ -9,6 +9,13 @@
 #ifndef LLDB_PLUGINS_SCRIPTINTERPRETER_PYTHON_LLDB_PYTHON_H
 #define LLDB_PLUGINS_SCRIPTINTERPRETER_PYTHON_LLDB_PYTHON_H
 
+// BEGIN FIXME
+// This declaration works around a clang module build failure.
+// It should be deleted ASAP.
+#include "llvm/Support/Error.h"
+static llvm::Expected<bool> *g_fcxx_modules_workaround;
+// END
+
 #include "lldb/Host/Config.h"
 
 // Python.h needs to be included before any system headers in order to avoid