Fix update_cc_test_checks.py --llvm-bin after D78478

Not passing --clang would result in a python exception after this change:
(TypeError: expected str, bytes or os.PathLike object, not NoneType)
because the --clang argument default was only being populated in the
initial argument parsing pass but not later on.
Fix this by adding an argparse callback to set the default values.

Reviewed By: vitalybuka, MaskRay

Differential Revision: https://reviews.llvm.org/D84511