[clang] [clang]fix ppcaching assertion gh186582 (PR #187370)

[via cfe-commits]

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Serosh-commits wrote:

> Your test doesn't seem to fail on trunk? https://godbolt.org/z/savrY1PYq

sry abt that i was trying to bookmark it so the crash only happens at global scope because Clang parses a(::template operator as a decl , leading to a mismatch between the parser and preprocessor cache inside a function treated as an expr so the issue doesn’t occur:/
i will update the test file to the original reproducer there it crashes in the trunk 

https://github.com/llvm/llvm-project/pull/187370