[clang] [clang] NFC: Add alias for std::pair<FileID, unsigned> used in SourceLocation (PR #145711)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LG from my side, but I would wait for another approval just to make sure people are on board with a relatively-large NFC refactoring.

Code style is a subjective matter, so having a little more backing from @AaronBallman or someone else looks appropriate.

https://github.com/llvm/llvm-project/pull/145711