Validate immutable variables

test/libsolidity/syntaxTests/immutable/increment.sol

@@ -0,0 +1,8 @@
+contract C {
+    uint immutable x = 3;
+    constructor() public {
+        x++;
+    }
+}
+// ----
+// TypeError: (74-75): 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.