[OpenMP][OMPD] GDB plugin code to leverage libompd to provide debugging

support for OpenMP programs.

This is 5th of 6 patches started from https://reviews.llvm.org/D100181
This plugin code, when loaded in gdb, adds a few commands like
ompd icv, ompd bt, ompd parallel.
These commands create an interface for GDB to read the OpenMP
runtime through libompd.

Reviewed By: @dreachem
Differential Revision: https://reviews.llvm.org/D100185