[PGO] Refactor Value Profiling into a plugin based oracle and create a well...

[PGO] Refactor Value Profiling into a plugin based oracle and create a well defined API for the plugins.

Summary: This PR creates a utility class called ValueProfileCollector that tells PGOInstrumentationGen and PGOInstrumentationUse what to value-profile and where to attach the profile metadata. It then refactors logic scattered in PGOInstrumentation.cpp into two plugins that plug into the ValueProfileCollector.

Authored By: Wael Yehia <wyehia@ca.ibm.com>

Reviewer: davidxl, tejohnson, xur

Reviewed By: davidxl, tejohnson, xur

Subscribers: llvm-commits

Tag: #llvm

Differential Revision: https://reviews.llvm.org/D67920

Patch By Wael Yehia <wyehia@ca.ibm.com>

llvm-svn: 373601