[PGO] Handle and report overflow during profile merge for all types of data

Summary: Surface counter overflow when merging profile data. Merging still occurs on overflow but counts saturate to the maximum representable value. Overflow is reported to the user.

Reviewers: davidxl, dnovillo, silvas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15547

llvm-svn: 255825