regression: Clang C++ miscompilation at -O2, please cherry pick https://github.com/llvm/llvm-project/pull/152913

Xcode 16.4 (but not older) and 26.0b5 miscompile/misoptimize some C++ code.

Details in upstream issue: https://github.com/llvm/llvm-project/issues/152824

Fixed upstream in: https://github.com/llvm/llvm-project/pull/152913

Please cherry-pick that fix.

(I hope this is the right place to file this, this seems to be the home of Apple's Clang fork?)

See also FB19522392.
