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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Did a better run-through again, so these are all thats left for me.  Thanks for the patience!

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