[clang] Move two utility functions into SourceManager

Summary: So we can keep that not-so-great logic in one place.

Reviewers: rsmith, aaron.ballman

Reviewed By: rsmith

Subscribers: nemanjai, kbarton, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D53837

llvm-svn: 345594