Read discriminators correctly from object file.

Summary:
This is the follow-up patch for http://reviews.llvm.org/D19436
* Update the discriminator reading algorithm to match the assignment algorithm.
* Add test to cover the new algorithm.

Reviewers: dnovillo, echristo, dblaikie

Subscribers: danielcdh, dblaikie, echristo, llvm-commits, joker.eph

Differential Revision: http://reviews.llvm.org/D19522

llvm-svn: 267945