Ifdefs, moved semantic to check, fixed error reporting for builtins

test/if_def_block.ink

@@ -0,0 +1,11 @@
+#add_define Alpha
+
+pixel main :: () {
+#if Env.Alpha {
+	alpha_color := float4(1, 0, 0, 1);
+	f := 2.0;
+} else {
+	color := float3(0, 0, 0);
+	g := 5.0;
+}
+}