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

[Ross Kirsling via cfe-commits]

rkirsling wrote:

Seems like I don't have the ability to add reviewers yet; pinging @cor3ntin and @AaronBallman for visibility.

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