Merge #7904

7904: Improved completion sorting r=JoshMcguigan a=JoshMcguigan

I was working on extending #3954 to apply completion scores in more places (I'll have another PR open for that soon) when I discovered that actually completion sorting was not working for me at all in `coc.nvim`. This led me down a bit of a rabbit hole of how coc and vs code each sort completion items.

Before this PR, rust-analyzer was setting the `sortText` field on completion items to `None` if we hadn't applied any completion score for that item, or to the label of the item with a leading whitespace character if we had applied any completion score. Completion score is defined in rust-analyzer as an enum with two variants, `TypeMatch` and `TypeAndNameMatch`. 

In vs code the above strategy works, because if `sortText` isn't set [they default it to the label](https://github.com/microsoft/vscode/commit/b4ead4ed665e1ce2e58d8329c682f78da2d4e89d). However, coc [does not do this](https://github.com/neoclide/coc.nvim/blob/e211e361475a38b146a903b9b02343551c6cd372/src/completion/complete.ts#L245).

I was going to file a bug report against coc, but I read the [LSP spec for the `sortText` field](https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_completion) and I feel like it is ambiguous and coc could claim what they do is a valid interpretation of the spec.

Further, the existing rust-analyzer behavior of prepending a leading whitespace character for completion items with any completion score does not handle sorting `TypeAndNameMatch` completions above `TypeMatch` completions. They were both being treated the same.

The first change this PR makes is to set the `sortText` field to either "1" for `TypeAndNameMatch` completions, "2" for `TypeMatch` completions, or "3" for completions which are neither of those. This change works around the potential ambiguity in the LSP spec and fixes completion sorting for users of coc. It also allows `TypeAndNameMatch` items to be sorted above just `TypeMatch` items (of course both of these will be sorted above completion items without a score). 

The second change this PR makes is to use the actual completion scores for ref matches. The existing code ignored the actual score and always assumed these would be a high priority completion item.

#### Before

Here coc just sorts based on how close the items are in the file.

![image](https://user-images.githubusercontent.com/22216761/110249880-46063580-7f2d-11eb-9233-91a2bbd48238.png)

#### After

Here we correctly get `zzz` first, since that is both a type and name match. Then we get `ccc` which is just a type match.

![image](https://user-images.githubusercontent.com/22216761/110249883-4e5e7080-7f2d-11eb-9269-a3bc133fdee7.png)


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

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