Add run-pass test suggested by @joshtriplett

Author: Aaron1011

src/test/ui/parser/if-attrs/gate-whole-expr.rs

@@ -0,0 +1,15 @@
+// run-pass
+
+fn main() {
+	let x = 1;
+
+	#[cfg(FALSE)]
+	if false {
+		x = 2;
+	} else if true {
+		x = 3;
+	} else {
+		x = 4;
+	}
+	assert_eq!(x, 1);
+}