Add 'thinlto_src_module' metadata to imported function

Added metadata to be able to make statistics on how many functions
that have been imported have been removed. Also module name might
be helpfull when debugging.

Reviewers: tejohnson, eraman

Subscribers: mehdi_amini, llvm-commits

Differential Revision: http://reviews.llvm.org/D21943

llvm-svn: 274668