[llvm] [Support] Introduce formatv variant of createStringError (PR #80493)

[Alex Langford via llvm-commits]

bulbazord wrote:

> This commit increases time to compile clang by 0.5% by adding an expensive include to a core header. The impact is large enough that we should make an effort to avoid it -- if all else fails, by putting this into a separate header.

I've not yet introduced any uses of this, so reverting should be cheap to do. I'll revert for now and figure something else out later.

https://github.com/llvm/llvm-project/pull/80493