[cmake] Add an option to build llvm with IR PGO

This adds an LLVM_ENABLE_IR_PGO option to enable building llvm and its
tools with IR PGO instrumentation.

Usage: -DLLVM_BUILD_INSTRUMENTED=On -DLLVM_ENABLE_IR_PGO=On (both
options must be enabled)

Differential Revision: https://reviews.llvm.org/D38066

llvm-svn: 313770