Calculate drop glue and show it on hover

Also fix the `needs_drop()` intrinsic.

Unions also need this information (to err if they have a drop-needing field), but this will come in a follow-up PR.

editors/code/package.json

@@ -1641,6 +1641,16 @@
                     }
                 }
             },
+            {
+                "title": "hover",
+                "properties": {
+                    "rust-analyzer.hover.dropGlue.enable": {
+                        "markdownDescription": "Whether to show drop glue information on hover.",
+                        "default": true,
+                        "type": "boolean"
+                    }
+                }
+            },
             {
                 "title": "hover",
                 "properties": {