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

[Louis Dionne via cfe-commits]

https://github.com/ldionne approved this pull request.

I'd rather keep the libc++ tests green while landing this PR. I tried something out, let's see if that works.

Otherwise LGTM, thanks for tackling this long-standing item on our TODO list :-)

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