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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

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

VLAs are always something we need to care about thanks to exposing them in C++. :-D We should add some test coverage to ensure the behavior is reasonable.

Other test cases to consider: lifetime around `this` in constructors and destructors, lifetime started via placement new/explicit destructor calls, and dangling references.

Also, we should update the status page, and what's the plan for the feature test macro?

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