[clang] [clang] Adding an API to create a `CXStringSet` from a Vector of `StringRef`s (PR #136773)

[Qiongsi Wu via cfe-commits]

qiongsiwu wrote:

> If the only use is in a downstream of Clang, do we need to upstream the changes at all? We could always upstream once there's an in-tree need for the functionality.

No, we don't _need_ to upstream. I don't have a strong feeling - I am upstreaming it because functionally this belongs to the `CXString` related methods, and it feels weird not having it upstream. Upstreaming it also has the benefit of allowing others to take advantage of it. 

Since it is hard to test let me move this downstream. We can upstream when it matures more. 

https://github.com/llvm/llvm-project/pull/136773