[clang] [Clang] Diagnose unsatisfied `std::is_assignable`. (PR #144836)

[Erich Keane via cfe-commits]

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

1 very minor nit on a rename, else LGTM. Please let others make sure they are happy before merging.

https://github.com/llvm/llvm-project/pull/144836