Minor Instr PGO code restructuring

1. Key constant values (version, magic) and data structures related to raw and 
   indexed profile format are moved into one centralized file: InstrProf.h.
2. Utility function such as MD5Hash computation is also moved to the common 
   header to allow sharing with other components in the future.
3. A header data structure is introduced for Indexed format so that the reader
   and writer can always be in sync.
4. Added some comments to document different places where multiple definition
   of the data structure must be kept in sync (reader/writer, runtime, lowering 
   etc).  No functional change is intended.

Differential Revision:  http://reviews.llvm.org/D13758

llvm-svn: 250638