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

[Erich Keane via cfe-commits]

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

This is fine to me, please give other reviewers another day to take a look before merging though.

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