Output error when forward referencing constants in inline assembly

test/libsolidity/syntaxTests/inlineAssembly/const_forward_reference.sol

@@ -0,0 +1,8 @@
+contract C {
+  function f() public pure {
+    assembly {
+      pop(add(add(1, 2), c))
+    }
+  }
+  int constant c = 1;
+}

test/libsolidity/syntaxTests/inlineAssembly/invalid/const_forward_reference.sol

@@ -0,0 +1,12 @@
+contract C {
+  function f() {
+    assembly {
+      c := add(add(1, 2), c)
+    }
+  }
+  int constant c = 0 + 1;
+}
+// ----
+// SyntaxError: (15-83): No visibility specified. Did you intend to add "public"?
+// TypeError: (71-72): Constant variables with non-literal values cannot be forward referenced from inline assembly.
+// TypeError: (51-52): Constant variables cannot be assigned to.