[UpdateTestChecks] Emit warning when invalid test paths

Summary:
Recently I ran into the following issue:

./update_test_checks.py /path/not-existing-file.ll

The script was silent and I was suprised why the real test file hadn't been updated.

Solution:
Emit warning if we detect this problem.



Reviewers: lebedev.ri, spatel, jdoerfert, nikic

Reviewed By: lebedev.ri, spatel, jdoerfert, nikic

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64572

llvm-svn: 365818