Allow $defined take declarations: $defined(int x = y)

Taking the address of a label would cause a crash. #2430

test/test_suite/compile_time_introspection/defined_decl.c3

@@ -0,0 +1,5 @@
+fn int main(String[] args)
+{
+	$defined(int a = 3.0);
+	$defined(int[*] b); // #error: Inferred array types can only be used in declarations
+}

test/test_suite/statements/label_addr.c3

@@ -0,0 +1,10 @@
+import std;
+fn int main(String[] args)
+{
+	switch FOO: (1)
+	{
+		case 1:
+			io::printfn("%p", &FOO); // #error: It is not possible to take the address of a label
+	}
+	return 0;
+}