[clang] Force an ICE when reentrantly reading a concept decl (PR #224677)

[Ian Petersen via cfe-commits]

ispeters wrote:

> Just out of curiosity, can't we detect such access uninitialized fields issue by tools like asan?

Not ASAN in particular—I tried it and it actually hid the bug rather than find it. I think ASAN doesn't help here because the memory being read before being initialized is owned by a bump allocator so it's properly allocated from the perspective of `malloc`. Claude seems to think MSAN might help, but I don't have a Linux machine handy to try so I didn't test the suggestion.

https://github.com/llvm/llvm-project/pull/224677