[memprof] Extend llvm-profdata to display MemProf profile summaries.

This commit adds initial support to llvm-profdata to read and print
summaries of raw memprof profiles.
Summary of changes:
* Refactor shared defs to MemProfData.inc
* Extend show_main to display memprof profile summaries.
* Add a simple raw memprof profile reader.
* Add a couple of tests to tools/llvm-profdata.

Differential Revision: https://reviews.llvm.org/D114286