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

[Nikita Popov via llvm-commits]

nikic 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.

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