Auto merge of #109760 - MaciejWas:struct-tuple-field-names-suggestion, r=jackh726

Better diagnostic when pattern matching tuple structs

Fixes #108284

When trying to pattern match a tuple struct we might get a flawed error message if there are missing fields. E.g.

```
let x = Foo(100, 200);
if let Foo { 0: bar } = x { ... }
```

Produces this error:

```
error[E0769]: tuple variant `Foo` written as struct variant
 --> hello.rs:5:12
  |
5 |     if let Foo { 0: foo } = x {
  |            ^^^^^^^^^^^^^^
  |
help: use the tuple variant pattern syntax instead
  |
5 |     if let Foo(_, _) = x {
  |               ~~~~~~
```

Which doesn't highlight that we can still use the struct syntax but we need to fill missing fields. This pr changes this error to:

```
error[E0027]: pattern does not mention field `1`
 --> hello.rs:5:12
  |
5 |     if let Foo { 0: foo } = x {
  |            ^^^^^^^^^^^^^^ missing field `1`
  |
help: include the missing field in the pattern
  |
5 |     if let Foo { 0: foo, 1: _ } = x {
  |                        ~~~~~~~~
help: if you don't care about this missing field, you can explicitly ignore it
  |
5 |     if let Foo { 0: foo, .. } = x {
  |                        ~~~~~~
```

Author: bors