Use the %clang_cc1 substitution consistently across the test suite,
replacing inline %clang -cc1 invocations, except for one Preprocessor
test where this is causing breakage. This is necessary to ensure that
additional parameters passed via %clang do not interfere with -cc1
that must always be passed as the first command-line argument.
Remove the additional substitution blocking %clang_cc1 use in Driver
tests. It has been added in 2013 and was supposed to prevent tests
calling clang -cc1 from being added to Driver. The state of the test
suite proves that it did not succeed at all.
But we _don't_ want to use %clang_cc1 in Driver tests; if we do, then we're not testing the driver! Those tests really belong somewhere else, like Frontend.