[clang] [clang] Fix partial specialization matching for NTTPs of placeholder type (PR #225239)

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

This is a "request changes" review, but I'd _really_ like @mizvekov and/or @cor3ntin to verify this is a correct suggestion, as I'm defining "correct" based on what we do somewhere else, and my use of `getParamTypeForDecl()` is based on the tried and true method of copying what we call elsewhere.

https://github.com/llvm/llvm-project/pull/225239