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

[Mital Ashok via cfe-commits]

MitalAshok wrote:

Hmm. On reading https://eel.is/c++draft/meta.const.eval#4 again, it says:

> *Remarks*: During the evaluation of an expression `E` as a core constant expression, a call to this function is ill-formed unless `p` points to an object that is usable in constant expressions or whose complete object's lifetime began within `E`.

So calling it inside constructors before the complete-object's lifetime has began (<https://eel.is/c++draft/basic.life#1.2>) should be ill-formed (instead of the current behaviour of `false`)

But I'm unsure about dangling pointers. The complete object's lifetime had begun, but it also ended. And https://cplusplus.github.io/CWG/issues/2822.html seems to make it implementation defined to use the pointer

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