[PGO] Introduce value profile data closure type.

The closure is designed to abstact away two types of value profile
data:
  - InstrProfRecord which is the primary data structure used to
    represent profile data in host tools (reader, writer, and profile-use)
  - value profile runtime data structure suitable to be used by C
    runtime library.
Both sources of data need to serialize to disk/memory-buffer in common
format: ValueProfData.

The abstraction allows compiler-rt's raw profiler writer to share
the same code with indexed profile writer.

llvm-svn: 254008