Only enable AddDiscriminator pass when -fdebug-info-for-profiling is true

Summary: AddDiscriminator pass is only useful for sample pgo. This patch restricts AddDiscriminator to -fdebug-info-for-profiling so that it does not introduce unecessary debug size increases for non-sample-pgo builds.

Reviewers: dblaikie, aprantl

Reviewed By: dblaikie

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D30220

llvm-svn: 295764