Revert commit 925a11a5f221 from llvm-project (by Nikolas Klauser):
[libc++] Simplify is_convertible GCC 13 has added __is_convertible(), so we don't need to keep the fallback implementation around. Reviewed By: #libc, Mordante Spies: Mordante, libcxx-commits Differential Revision: https://reviews.llvm.org/D157939
This should allow buildworld with gcc 12 to work again.
PR: 276104
MFC after: 1 month