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

[Demetrios Chiuratto Agourakis via cfe-commits]

agourakis82 wrote:

Updated with the requested changes:
- Added the original test case from #225335.
- Added test coverage for `-fexperimental-new-constant-interpreter` with appropriate diagnostic prefixes.
- Added a release note under `Bug Fixes to Compiler Builtins` in `clang/docs/ReleaseNotes.md`.

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