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

[Daniil Fukalov via llvm-commits]

dfukalov wrote:

> 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 And if more macros get used in the file or get removed, then the comment is out of date.

Well, I added such comments since during cleanups it is not always obvious why such headers are included. And clangd time to time reports these as unused. Also, we already have here and there same comments.

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