Invoke add-discriminator at -g0 -fsample-profile

Summary: -fsample-profile needs discriminator, which will not be added if built with -g0. This patch makes sure the discriminator is added for sample-profile at -g0. A followup patch will be send out to update clang tests.

Reviewers: davidxl, dblaikie, echristo, dnovillo

Subscribers: mehdi_amini, probinson, llvm-commits

Differential Revision: https://reviews.llvm.org/D25132

llvm-svn: 283565