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

[Simon Tatham via cfe-commits]

statham-arm wrote:

> It is fine to add `fixes https://github.com/llvm/llvm-project/issues/123864` to the Description so that the issue is automatically closed.

Yes, good point! When I suggested an improved description, I left out that part, which is the one thing @Zhenhang1213 _didn't_ forget in the original. Sorry.

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