[clang] [Clang] [AST] Fix placeholder return type name mangling for MSVC 1920+ / VS2019+ (PR #102848)

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MaxE707 wrote:

> We're hitting "Invalid type expected" and "UNREACHABLE executed at clang/lib/AST/MicrosoftMangle.cpp:2551!" after this.
> 
> See https://crbug.com/359904717#comment2 for a reproducer.
> 
> I'll revert back to green until it can be fixed.

Thanks for the revert. I'll take a look as soon as I got some free time which will probably be tomorrow :).

https://github.com/llvm/llvm-project/pull/102848