[clang] [clang][SourceManager] Use `getFileLoc` when computing `getPresumedLoc` (PR #168402)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

Precommit CI found some relevant failures; but I'm not certain `awk` is going to be more portable (as best I can tell, this will be the first use of `awk` in any Clang tests). It's hard to understand what the test file is testing, but I'm also coming from a more Windows-centric background.

https://github.com/llvm/llvm-project/pull/168402