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

[Dana Jansens via cfe-commits]

danakj wrote:

The subspace library also provides mechanisms [for querying](https://suslib.cc/sus-mem-TriviallyRelocatable.html) and marking types trivially relocatable, and relies on the compiler's `__is_trivially_relocatable` as a signal as well. It then optimizes containers and operations internally based on its answer, and allows client code to do the same. Getting `__is_trivially_relocatable` correct here would be very appreciated.

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