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

[Erich Keane via cfe-commits]

erichkeane wrote:

> Thanks! Could I also have one of you merge this for me once everybody's happy? 🙏

Ah, a little late in my day to be doing that (the 'click merge and take off' strat is one fraught with error :) ), but if no one else has by my morning, I'll do so.

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