[PGO] More comments how function pointers for indirect calls are mapped

to function names

Summary:
Hopefully this will make it easier for the next person to figure all
this out...

Reviewers: bogner, davidxl

Subscribers: davidxl, cfe-commits

Differential Revision: http://reviews.llvm.org/D18489

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264678 91177308-0d34-0410-b5e6-96231b3b80d8