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

[Amirreza Ashouri via cfe-commits]

AMP999 wrote:

> We (users & implementers) have all collectively identified trivial relocatability as an important optimization for vector-like containers, and we're all looking for a solution to that. It feels like this attempt to standardize this type trait is getting in the way of our collective ability to add vendor extensions that were not necessarily intended to become standards-track proposals.

Since people are asking for these semantics in Clang's vendor extension, how would people feel about proceeding in this direction for now, and adding a second type trait, named something like `__is_cpp_trivially_relocatable(T)`, for the P2786 version?

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