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

[Reid Kleckner via cfe-commits]

rnk wrote:

So, I was completely unaware that trivial relocatability had been picked up at all by WG21. Since the beginning of `trivial_abi`, I we were solidly in the vendor-extension space trying to build non-standard but practical solutions to real world problems, like the fact that we couldn't pass `unique_ptr` in registers. 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.

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