Introduce -print-changed=[diff | diff-quiet] which show changes in patch-like format

Summary:
Introduce base classes that hold a textual represent of the IR
based on basic blocks and a base class for comparing this
representation.  A new change printer is introduced that uses these
classes to save and compare representations of the IR before and after
each pass.  It only reports when changes are made by a pass (similar to
-print-changed) except that the changes are shown in a patch-like format
with those lines that are removed shown in red prefixed with '-' and those
added shown in green with '+'.  This functionality was introduced in my
tutorial at the 2020 virtual developer's meeting.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By: aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D91890