Restore "[ThinLTO] Serialize the Module SourceFileName to/from LLVM assembly"

This restores commit 264869, with a fix for windows bots to properly
escape '\' in the path when serializing out. Added test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264884 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed