5846: Add references to fn args during completion r=matklad a=adamrk
When completing a function call, if there is an argument taken as a ref or mut ref which matches the name and type of a variable in scope, we will insert a `&` or `&mut` when filling in the function arguments. This addresses https://github.com/rust-analyzer/rust-analyzer/issues/5449.
E.g.
```rust
fn foo(x: &i32) {}
fn main() {
let x = 5;
foo # completing foo here generates `foo(&x)` now instead of `foo(x)`
}
```
Co-authored-by: adamrk <ark.email@gmail.com>
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