[clang] [Clang] Handle references to concept in Subst* nodes (PR #199671)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Some tests are failing, mostly diag issues that i know how to fix.
The crash in `clang/test/SemaTemplate/concepts-lambda.cpp` confuses me though, depth issue.

Before i refine the patch, I'd like feedback on the approach @mizvekov @zyn0217 @erichkeane.
There are probably easier way - hackier way to get the args of the concept under evaluation, but this has the benefit of not introducing a new way to do things - ie it is fairly consistent.

WDYT?


https://github.com/llvm/llvm-project/pull/199671