[ThinLTO] Add option to dump value name to GUID mapping

Summary:
Useful for debugging since we lose this correlation after the permodule
summary/VST is read and until we later materialize source modules in the
function importer.

Reviewers: joker.eph

Subscribers: llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D18555

llvm-svn: 265327