Merge #6331 #6342

6331: correct hover text for items with doc attribute with raw strings r=matklad a=JoshMcguigan

Fixes #6300 by improving the handling of raw string literals in attribute style doc comments.

This still has a bug where it could consume too many `"` at the start or end of the comment text, just as the original code had. Not sure if we want to fix that as part of this PR or not? If so, I think I'd prefer to add a unit test for either the `as_simple_key_value` function (I'm not exactly sure where this would belong / how to set this up) or create a `fn(&SmolStr) -> &SmolStr` to unit test by factoring out the `trim` operations from `as_simple_key_value`. Thoughts on this? 

6342: Shorter dependency chain r=matklad a=popzxc

Continuing implementing suggestions from the `Completion refactoring` zulip thread.

This PR does the following:

- Removes dependency of `completions` on `assists` by moving required functionality into `ide_db`.
- Moves completely `call_info` crate into `ide_db` as it looks like it fits perfect there.
- Adds a bunch of new tests and docs.
- Adds the re-export of `base_db` to the `ide_db` and removes direct dependency on `base_db` from other crates.

The last point is controversial, I guess, but I noticed that in places where `ide_db` is used, `base_db` is also *always* used. Thus I think the dependency on the `base_db` is implied by the fact of `ide_db` interfaces, and thus it makes sense to just provide `base_db` out of the box.


Co-authored-by: Josh Mcguigan <[email protected]>
Co-authored-by: Igor Aleksanov <[email protected]>