[clang] [Clang] Implement the core language parts of P2786 - Trivial relocation (PR #127636)

[Erich Keane via cfe-commits]

erichkeane wrote:

I think the promise to do the deprecation 'soon' (and by 'soon' it needs to be before the next release IMO, else we need to revert this, as releasing this without the diagnostic is, IMO, not tennable) is acceptable.

https://github.com/llvm/llvm-project/pull/127636