[llvm] [NFC] Add explicit #include llvm-config.h where its macros are used. (PR #106621)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

I'm not sure the comments after the `# include` are necessary?

They don't seem to provide much useful information. If I'm looking for unused headers, I'd probably be using `clangd` with IWYU flags, which wouldn't pick these up given they are used. And if more macros get used in the file or get removed, then the comment is out of date.

https://github.com/llvm/llvm-project/pull/106621