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

[Nikita Popov via llvm-commits]

nikic wrote:

I think it's ok to add the comment *for defines only*, because there is the risk of dropping the include without breaking the build, just silently changing behavior (which is what happened in https://github.com/llvm/llvm-project/pull/104825.)

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