[ProfileData] Add unit test infrastructure for sample profile reader/writer

Summary:
Adds support for in-memory round-trip of sample profile data along with basic
round trip unit tests. This will also make it easier to include unit tests for
future changes to sample profiling.

Reviewers: davidxl, dnovillo, silvas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15211

llvm-svn: 255264