Merge pull request #11482 from ethereum/fixUnreachable2

Fix control-flow handling of modifiers without body.

test/libsolidity/syntaxTests/controlFlow/modifiers/implemented_without_placeholder.sol

@@ -0,0 +1,8 @@
+abstract contract A {
+    function f() public view mod {
+        require(block.timestamp > 10);
+    }
+    modifier mod() virtual { }
+}
+// ----
+// SyntaxError 2883: (129-132): Modifier body does not contain '_'.

test/libsolidity/syntaxTests/controlFlow/modifiers/non_implemented_modifer.sol

@@ -0,0 +1,6 @@
+abstract contract A {
+    function f() public view mod {
+        require(block.timestamp > 10);
+    }
+    modifier mod() virtual;
+}