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

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zmodem 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.

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