[LLVM-C] Expose the "Add Discriminators" Pass To LLVM-C

Summary: Add bindings to create a wrapped "Add Discriminators" pass.  Now that we have debug info support, this is a handy transform to have.

Reviewers: whitequark, deadalnix

Reviewed By: whitequark

Subscribers: dblaikie, aprantl, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58624

llvm-svn: 356272