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

[Ulrich Weigand via cfe-commits]

uweigand wrote:

For SystemZ the correct value is 256.   In general I agree it makes sense to look at the GCC implementation as a source of reasonable values.   Also, I think there probably should be no generic default value at all - it there is no platform-specific value known, it seems better to not define those values rather than define them to some incorrect value ...

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