Add show and merge tools for sample PGO profiles.

Summary:
This patch extends the 'show' and 'merge' commands in llvm-profdata to handle
sample PGO formats. Using the 'merge' command it is now possible to convert
one sample PGO format to another.

The only format that is currently not working is 'gcc'. I still need to
implement support for it in lib/ProfileData.

The changes in the sample profile support classes are needed for the
merge operation.

Reviewers: bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D6065

llvm-svn: 221032