Add the config needed to get rust-analyzer working on src/bootstrap (rust-lang#1381)

Author: jyn514

src/doc/rustc-dev-guide/src/building/suggested.md

@@ -46,6 +46,10 @@ you can write: <!-- date: 2022-04 --><!-- the date comment is for the edition be
         "--message-format=json"
     ],
     "rust-analyzer.rustc.source": "./Cargo.toml",
+    "rust-analyzer.linkedProjects": [
+        "Cargo.toml",
+        "src/bootstrap/Cargo.toml"
+    ]
 }
 ```