[clang] [C++20] Fix crash with invalid concept requirement (PR #138877)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Does this also fix: #138823

I don't believe so, at least from my testing on godbolt (which should have my changes, from the git hash they have with --version).

https://github.com/llvm/llvm-project/pull/138877