Check for use of modifiers in invalid contexts

test/libsolidity/syntaxTests/modifiers/use_in_invalid_context.sol

@@ -0,0 +1,9 @@
+contract test {
+    modifier mod() { _; }
+
+    function f() public {
+        mod  ;
+    }
+}
+// ----
+// TypeError: (77-80): Modifier can only be referenced in function headers.