[clang] [clang][Sema] Track trivial-relocatability as a type trait (PR #84621)

[via cfe-commits]

sjanel wrote:

The [amc](https://github.com/AmadeusITGroup/amc) library defining drop-in replacement for vector-like containers handling trivially relocatability optimizations would love to see this kind of improvement, as currently the users need to manually tell the compiler which types are OK to consider as trivially relocatable. I would be glad to test it!

https://github.com/llvm/llvm-project/pull/84621