[clang] [clang] fix crash substituting decltype parameters during partially ordered deduction (PR #214130)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

CC'ing @zwuis and @erichkeane as Aaron suggested you folk as well as people who might have time to look at this. This is still a draft as I'm unfamiliar with this code and have repeatedly managed to make PRs that are only marginally better than the slop generated ones.

I've tried to implement what @mizvekov had said was the correct fix, but again I'm a muppet.

https://github.com/llvm/llvm-project/pull/214130