Validate immutable variables

test/libsolidity/syntaxTests/immutable/selector.sol

@@ -0,0 +1,12 @@
+contract C {
+    uint immutable x;
+    constructor() public {
+        x = 3;
+        this.readX.selector;
+    }
+
+    function readX() external view returns(uint) { return x; }
+}
+// ----
+// Warning: (85-104): Statement has no effect.
+// Warning: (85-89): "this" used in constructor. Note that external functions of a contract cannot be called while it is being constructed.