Suggest replacing typeof(...) with an actual type

This commit suggests replacing typeof(...) with an actual type of "...",
for example in case of `typeof(1)` we suggest replacing it with `i32`.

If the expression
- Is not const (`{ let a = 1; let _: typeof(a); }`)
- Can't be found (`let _: typeof(this_variable_does_not_exist)`)
- Or has non-suggestable type (closure, generator, error, etc)
we don't suggest anything.

compiler/rustc_error_messages/locales/en-US/diagnostics.ftl

@@ -62,6 +62,7 @@ typeck-functional-record-update-on-non-struct =
 
 typeck-typeof-reserved-keyword-used =
     `typeof` is a reserved keyword but unimplemented
+    .suggestion = consider replacing `typeof(...)` with an actual type
     .label = reserved keyword
 
 typeck-return-stmt-outside-of-fn-body =