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

[via cfe-commits]

GeneralK1ng wrote:

> Thanks for the fix! Does this fix #115513 and #130620 as well?

Thanks for checking!

I verified both reproducers locally with this patch applied:

- #115513: the reproducer no longer asserts; it now just emits diagnostics and exits normally.
- #130620: the reproducer no longer triggers the `ExprRequirement::getExpr()` assertion either; it completes without crashing (only a warning is emitted).

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