SourceManager: Open code isInMainFile.

- We really shouldn't compute line numbers for every file that is asked if it's
  the main file, it destroys the lazy computation.
- Invalid locations are no longer accounted to the main file, no other
  functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@191535 91177308-0d34-0410-b5e6-96231b3b80d8