Tests: Change substitution to automagically use 'clang -cc1' instead of...

Tests: Change substitution to automagically use 'clang -cc1' instead of 'clang-cc'. I still plan to eventually rewrite the tests to clarify the magic (I will probably actually rewrite them to %clang so it is more obvious this is a variable that gets substituted).

llvm-svn: 91173