[clang] [ARM] Introduce -mtp=auto and make it the default (PR #128728)

[Peter Smith via cfe-commits]

smithp35 wrote:

A couple of things I forgot to mention:
* Please can you update the in progress ReleaseNotes in https://github.com/llvm/llvm-project/blob/main/clang/docs/ReleaseNotes.rst as this is a potentially breaking change.
* It is fine to add `fixes https://github.com/llvm/llvm-project/issues/123864` to the Description so that the issue is automatically closed.

https://github.com/llvm/llvm-project/pull/128728