Reapply "Add update_test.py script."

Originally committed in r261899 and reverted in r262202 due to failing
in out-of-LLVM tree builds.

Replace the use of LLVM_TOOLS_BINARY_DIR by LLVM_TOOLS_DIR which exists
in both, in-tree and out-of-tree builds.

Original commit message:
The script updates a lit test case that uses FileCheck using the actual
output of the 'RUN:'-lines program. Useful when updating test cases due
to expected output changes and diff'ing expected and actual output.

llvm-svn: 262227