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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Sorry, I know nothing about it. But it looks to me it's to match with GCC, why don't borrow the value from GCC as a beginning?

Because the values depend on the target and the compiler options picked. I don't have that information myself, and given how many targets and options we support, I am hoping target owners can help rather than having to guess.

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