Support DWARF discriminators in object streamer.

Summary:
This adds support for emitting DWARF path discriminator values in
the object streamer. It also changes the DWARF dumper to show
discriminator values in the line table output.

Reviewers: echristo

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2794

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201427 91177308-0d34-0410-b5e6-96231b3b80d8