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

[Sergej Salnikov via cfe-commits]

SergejSalnikov wrote:

#AaronBallman, I've created another PR with a fix. Could you please run the failed targets to verify that the fixed version works?

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