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

Summary:
This change serializes out and in the SourceFileName to LLVM assembly
so that it is preserved through "llvm-dis | llvm-as". This is
necessary to ensure that the global identifiers created for local values
in the module summary index are the same even if the bitcode is
streamed out and read back from LLVM assembly.

Serializing the summary itself to LLVM assembly is in progress.

Reviewers: joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D18588

llvm-svn: 264869