[clang] [Clang] Defer the instantiation of explicit-specifier until constraint checking completes (PR #70548)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik approved this pull request.

I just want to make sure we have enough testing in place for this change but overall I think I am happy.

https://github.com/llvm/llvm-project/pull/70548