Instroduce a template file to define InstrPGO core data structures.

Changing PGO data format layout can be a pain. Many different places need
to be touched and kept in sync. Failing to do so usually results in errors
very time consuming to debug.

This file is intended to be the master file that defines the layout of the
core runtime data structures. Currently only two structure is covered: Per
function ProfData structure and the function record structure used in
coverage mapping.

No client code has been made yet, so this commit is NFC.

llvm-svn: 250574