Improve the forward declare protocols test.

Make it not fail to build with older compilers and make it test the
thing that should be true with the new ABI.

Test/ForwardDeclareProtocol.m

@@ -3,5 +3,12 @@
 
 Protocol *getProtocol(void)
 {
+// Don't try to compile this on known-broken compilers.
+#if !defined(__clang__)
 	return @protocol(P);
+#elif __clang_major__ > 6
+	return @protocol(P);
+#else
+	return 0;
+#endif
 }

Test/ForwardDeclareProtocolAccess.m

@@ -9,9 +9,12 @@ Protocol *getProtocol(void);
 
 int main(void)
 {
-	//Protocol *p1 = @protocol(P);
+	Protocol *p1 = @protocol(P);
 	Protocol *p2 = getProtocol();
-	//assert(protocol_isEqual(p1, p2));
-	//assert(p1 == p2);
+	assert(protocol_isEqual(p1, p2));
+#ifdef GS_RUNTIME_V2
+	// With the new ABI, these should be precisely the same object.
+	assert(p1 == p2);
+#endif
 	return 0;
 }