[clang] [clang][MSVC] Correct mangling of thread-safe static initialization variables. (PR #85300)

[Tom Honermann via cfe-commits]

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@@ -47,6 +47,12 @@ C++ Specific Potentially Breaking Changes
 
 ABI Changes in This Version
 ---------------------------
+- Fixed Microsoft name mangling of implicitly defined variables used for thread
+  safe static initialization of static local variables. This change resolves
+  incompatibilities with code compiled by MSVC but might introduce
+  incompatibilities with code compiled by earlier versions of Clang when an
+  inline member function that contains a static local variable with a dynamic
+  initializer is declared with ``__declspec(dllimport)``. (#GH83616).
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tahonermann wrote:

Per discussion in https://github.com/llvm/llvm-project/issues/83616, I did not tie the fix to the `-fclang-abi-compat=` option. I checked for uses of `LangOptions::ClangABI` and while I see many such uses in `ItaniumMangle.cpp`, there are currently none in `MicrosoftMangle.cpp`.

https://github.com/llvm/llvm-project/pull/85300