[clang] [clang] Consistently cache failed constraint normalization (PR #227086)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

The summary was a bit confusing and I had to read it a few times to get it.

I think the main issues was the seperation between the bug description and the proposed fix.

I would have said something like:

"The proposed fix is to instead, cache failed substitution as nullptr, like a failed normalization.

As a consequences we don't require the diagnostics that were only emitted because the second lookup continued"

The splits the summary narrative more clearly into problem/solution/other fixes. Clear summaries mean easier and faster review.

https://github.com/llvm/llvm-project/pull/227086