diagnostics: mismatched pattern field: suggest ".." instead of "_"

[matthiaskrgr]

error[E0023]: this pattern has 1 field, but the corresponding tuple variant has 3 fields
   --> clippy_lints/src/needless_borrow.rs:50:20
    |
50  |             if let ty::Ref(_) = cx.typeck_results().expr_ty(inner).kind() {
    |                    ^^^^^^^^^^ expected 3 fields, found 1

trying to ignore the inner fields with ty::Ref(_) does not work because it has 3 fields.
The message could suggest Ref(..) if there are multiple fields and it thinks we don't want to bind any:

error[E0023]: this pattern has 1 field, but the corresponding tuple variant has 3 fields
   --> clippy_lints/src/needless_borrow.rs:50:20
    |
50  |             if let ty::Ref(_) = cx.typeck_results().expr_ty(inner).kind() {
    |                    ^^^^^^^^^^ expected 3 fields, found 1
    = help: Use `ty::Ref(..)` to ignore any inner fields

[matthiaskrgr]

@rustbot modify label: +A-diagnostics