Expose -mllvm -accurate-sample-profile to clang.

Summary: With accurate sample profile, we can do more aggressive size optimization. For some size-critical application, this can reduce the text size by 20%

Reviewers: davidxl, rsmith

Reviewed By: davidxl, rsmith

Subscribers: mehdi_amini, eraman, sanjoy, cfe-commits

Differential Revision: https://reviews.llvm.org/D37091

llvm-svn: 311707