[PGO] Add flags to control IRPGO warnings.

Currently there is no reasonable way to control the warnings in the 'use' phase
of the IRPGO pass. This is problematic because the output can be somewhat
spammy. This patch adds some flags which allow us to optionally disable these
warnings. The current upstream behavior will remain the default.

Patch by Jake VanAdrighem (jvanadrighem@gmail.com)

Differential Revision: http://reviews.llvm.org/D20195

llvm-svn: 269437