Validate immutable variables

test/libsolidity/syntaxTests/immutable/ctor_modifier_reading.sol

@@ -0,0 +1,14 @@
+contract C {
+    uint immutable x;
+    constructor() readX public {
+        x = 3;
+    }
+
+    modifier readX() {
+        _; f(x);
+    }
+
+    function f(uint a) internal pure {}
+}
+// ----
+// TypeError: (126-127): Immutable variables cannot be read during contract creation time, which means they cannot be read in the constructor or any function or modifier called from it.