[clang] [C++17] Support __GCC_[CON|DE]STRUCTIVE_SIZE (PR #89446)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

@AaronBallman thanks for picking this up! I agree with @cor3ntin and @ldionne that we should provide the macros unconditionally. This brings us one step closer to finally making libc++ fully conforming in C++17. With the PSTL finally making steady progress, we're probably only missing a few audits and the math special functions to be fully conforming.

https://github.com/llvm/llvm-project/pull/89446