Resolve completions properly (#18212)

Related to https://github.com/rust-lang/rust-analyzer/pull/18167

* Declare more completion item fields in the client completion resolve
capabilities
* Do resolve completions even if their docs are present
* Instead, do not resolve completions that could not be resolved when
handling the remote client resolve requests
* Do replace the old lsp completion data with the resolved one

Release Notes:

- Improved completion resolve mechanism