[clang] [Clang] Avoid crash when expression requirements are substitution failures (PR #176896)

[Thibault Monnier via cfe-commits]

https://github.com/Thibault-Monnier approved this pull request.

LGTM after you link all the fixed issues in the PR description (e.g. "Fixes #.")

https://github.com/llvm/llvm-project/pull/176896