As described in #69994, using the escape hatch makes us non-conforming in C++20 due to incorrect constexpr-ness. It also leads to bad diagnostics as reported by #63900. We discussed the issue in the libc++ monthly meeting and we agreed that we should deprecate the macro in LLVM 18, and then remove it in LLVM 19 since it causes too many problems. This patch does the first part of this -- it deprecates the macro. Fixes #69994 Fixes #63900 Partially addresses #75975