Reland "A new hidden option exec-on-ir-change=exe that calls exe each time IR changes"

Summary:
This relands commit dff0e8b4.  The test is now
guarded with a lit.local.cfg that ensures /bin/cat is available.  Also, the
code has been updated to match changes made to relevant code.

Author: Jamie Schmeiser <schmeise@ca.ibm.com>
Reviewed By:aeubanks (Arthur Eubanks)
Differential Revision: https://reviews.llvm.org/D110776