Explicitly reference a protocol in a test.

With the old ABI, we generated a copy of every protocol in every
compilation unit that declared it.  We now emit protocols only if they
are referenced (and have the linker deduplicate them).

This test was previously failing with the Apple runtime.

Test/ProtocolCreation.m

@@ -7,6 +7,7 @@
 
 int main(void)
 {
+	Protocol *force_reference = @protocol(Test2);
 	Protocol *p = objc_allocateProtocol("Test");
 	protocol_addMethodDescription(p, @selector(someMethod), "@:", YES, NO);
 	assert(objc_getProtocol("Test2"));