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

This reverts commit r264869. I am seeing Windows bot failures due to the
"\" in the path being mishandled at some point (seems to be interpreted
wrongly at some point and llvm-as | llvm-dis is yielding some junk
characters). Need to investigate.

llvm-svn: 264871