[clang] [clang][NFC] add static for internal linkage function and extract common function to header file (PR #109436)

[Chris B via cfe-commits]

llvm-beanz wrote:

I think we should separate adding the internal linkage (missing `static` keywords) from the other refactoring since they're unrelated.

Adding the missing `static` keywords is all good. I'm not sure I agree with the other refactoring.

https://github.com/llvm/llvm-project/pull/109436