Update discriminator assignment algorithm in clang assembler.

Summary: The clang assembler assumes that the discriminator remains the same when there is source line change. The correct behavior is that when there is line change, discriminator will automatically reset to 0.

Reviewers: dnovillo, davidxl, echristo

Subscribers: echristo, llvm-commits

Differential Revision: http://reviews.llvm.org/D19436

llvm-svn: 267226