Exclude non-identifier aliases from completion filtering text
When building `CompletionItem`s, this excludes aliases that aren't valid identifiers from the "lookup" text used to filter completions in the LSP client. Including them results in weird completion filtering behavior e.g. `Partial>` matching a completion for the `PartialOrd` trait because it has a doc alias of ">".
Closes#14692
2f2cf21da5