Fix bug 19437 - Only add discriminators for DWARF 4 and above.

Summary:
This prevents the discriminator generation pass from triggering if
the DWARF version being used in the module is prior to 4.

Reviewers: echristo, dblaikie

CC: llvm-commits

Differential Revision: http://reviews.llvm.org/D3413

llvm-svn: 206507