[clang] [Clang] Add __builtin_is_within_lifetime to implement P2641R4's std::is_within_lifetime (PR #91895)

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https://github.com/cor3ntin commented:

This looks great.
Most of the changes to DiagnosticASTKinds.td could be done as a nice NFC cleanup PR.
My only real concern is the way we reject function pointer.
We also might want tests for member function pointers.

Are VLAs something we need to care about here @AaronBallman ?

We are missing release note + change to LanguageExtentions.rst

https://github.com/llvm/llvm-project/pull/91895