[llvm-profdata] Support JSON as as an output-only format

This patch teaches llvm-profdata to output the sample profile in the
JSON format.  The new option is intended to be used for research and
development purposes.  For example, one can write a Python script to
take a JSON file and analyze how similar different inline instances of
a given function are to each other.

I've chosen JSON because Python can parse it reasonably fast, and it
just takes a couple of lines to read the whole data:

  import json
  with open ('profile.json') as f:
    profile = json.load(f)

Differential Revision: https://reviews.llvm.org/D130944