[clang] [llvm] Remove some "DeprecatedSmallSet is deprecated" warnings from the build (PR #155407)

[David Sherwood via llvm-commits]

david-arm wrote:

> Does this resolve _all_ of the warnings, or just some of them? If there are still warnings it is probably better to revert the deprecation change entirely

Only some of the warnings - I still see a lot of warnings coming from the inclusion of SmallSet.h in clang/include/clang/Basic/TargetInfo.h, but that header genuinely requires the SmallSet class.

I'm happy to revert the deprecation change - I just wasn't sure if I was the only person seeing it or not!

https://github.com/llvm/llvm-project/pull/155407