update_any_test_checks.py: run from any dir

Allow running llvm/utils/update_any_test_checks.py from working
directories other llvm/utils/.

Fixes:
$ ./llvm/utils/update_any_test_checks.py path/to/foo.ll
utils/update_llc_test_checks.py: not found (used in path/to/foo.ll)

Reviewed By: arichardson

Differential Revision: https://reviews.llvm.org/D140586