[PM] Add support for sample PGO in the new pass manager (clang-side)

Summary: This implements the clang bits of https://reviews.llvm.org/D34720, and add corresponding test to verify if it worked.

Reviewers: chandlerc, davidxl, davide, tejohnson

Reviewed By: chandlerc, tejohnson

Subscribers: tejohnson, sanjoy, mehdi_amini, eraman, cfe-commits

Differential Revision: https://reviews.llvm.org/D34721

llvm-svn: 306764