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

[Phoebe Wang via cfe-commits]

phoebewang 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?

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