Allow global struct definitions.

test/libsolidity/syntaxTests/structs/global_struct_shadowing.sol

@@ -0,0 +1,10 @@
+struct S { uint a; }
+contract C {
+    struct S { address x; }
+    function f() public view {
+        S memory s = S(address(this));
+        s;
+    }
+}
+// ----
+// Warning: (38-61): This declaration shadows an existing declaration.