[UpdateTestChecks] Emit warning when invalid test paths
Recently I ran into the following issue:
The script was silent and I was suprised why the real test file hadn't been updated.
Emit warning if we detect this problem.
Reviewers: lebedev.ri, spatel, jdoerfert, nikic
Reviewed By: lebedev.ri, spatel, jdoerfert, nikic
Subscribers: jdoerfert, llvm-commits
Differential Revision: https://reviews.llvm.org/D64572