Switch tests in test/Headers to use %clang_cc1 rather than %clang. There
is no need to go through the driver indirection here, and it clutters
things up as dependencies can sneak in for specific things the driver is
doing.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@191107 91177308-0d34-0410-b5e6-96231b3b80d8