[clang] [llvm] [LLVM][AArch64] Add ASM constraints for reduced GPR register ranges. (PR #70970)

[Paul Walker via cfe-commits]

paulwalker-arm wrote:

* Rebased.
* Extended coverage to include all typical scalar types.
* Updated the LangRef to document the new constraints.
* Added an entry to the release note.

https://github.com/llvm/llvm-project/pull/70970