Do not add discriminator encoding for debug intrinsics.

Summary: There are certain requirements for debug location of debug intrinsics, e.g. the scope of the DILocalVariable should be the same as the scope of its debug location. As a result, we should not add discriminator encoding for debug intrinsics.

Reviewers: dblaikie, aprantl

Reviewed By: aprantl

Subscribers: JDevlieghere, aprantl, bjope, sanjoy, llvm-commits

Differential Revision: https://reviews.llvm.org/D39343

llvm-svn: 316703