[clang] [clang] Stop claiming __builtin_assume_separate_storage is constexpr (PR #225515)

[Demetrios Chiuratto Agourakis via cfe-commits]

agourakis82 wrote:


@Fznamznon Could you take another look at `3e06b148dd4e`? The requested original #225335 reproducer, new-interpreter coverage, and release note are included.

Validation with Clang and FileCheck built from that exact head, with assertions enabled:

- Both checked-in SemaCXX RUN commands pass, including the original lambda reproducer.
- The builtin remains available and `noexcept`, while `__has_constexpr_builtin` is false.
- The existing CodeGen/FileCheck test passes, including argument side effects.
- A controlled rebuild restoring only the original builtin metadata makes the classic diagnostic test and constexpr-capability check fail as expected. Its CodeGen test still passes and emits identical IR.
- Restoring the candidate and rebuilding makes all focused checks pass again; the restored compiler is byte-identical to the initial candidate.

These were direct native test invocations, not a full `check-clang` run. This patch withdraws an unsupported constexpr capability; it does not implement constant evaluation.

Validation and this follow-up were prepared with Codex assistance.


https://github.com/llvm/llvm-project/pull/225515