This adds a lit plugin wrapping the core of update-verify-tests. When
lit is invoked with --update-tests it will call the plugin, which checks
if the failure is due to a -verify mismatch. If it is, it tries to
repair the test. If the source file was originally created by
split-file, the changes are propagated to the parent file.
No tests are added, because I don't want to run nested llvm-lit tests
in Swift's test suite, but the core functionality is tested through
update-verify-tests.py.
