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

[Louis Dionne via cfe-commits]

ldionne wrote:

> > I'd rather keep the libc++ tests green while landing this PR. I tried something out, let's see if that works.
> 
> It looks like some further adjustments may be needed as some stage1 builders are failing:
> 
> ```
> [...]
> ```

Ah, yes, we're using the nightly clang which is labeled as clang-19 but doesn't contain the change yet. I'll mark the test as `UNSUPPORTED` on all Clang flavors for now (which is how the test should have been marked), and we will fix that in the patch that implements the library feature.


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