Point at enum definition when match patterns are not exhaustive

```
error[E0004]: non-exhaustive patterns: type `X` is non-empty
 --> file.rs:9:11
  |
1 | / enum X {
2 | |     A,
  | |     - variant not covered
3 | |     B,
  | |     - variant not covered
4 | |     C,
  | |     - variant not covered
5 | | }
  | |_- `X` defined here
...
9 |       match x {
  |             ^
  |
  = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms

error[E0004]: non-exhaustive patterns: `B` and `C` not covered
  --> file.rs:11:11
   |
1  | / enum X {
2  | |     A,
3  | |     B,
4  | |     C,
   | |     - not covered
5  | | }
   | |_- `X` defined here
...
11 |       match x {
   |             ^ patterns `C` not covered
```

When a match expression doesn't have patterns covering every variant,
point at the enum's definition span. On a best effort basis, point at the
variant(s) that are missing. This does not handle the case when the missing
pattern is due to a field's enum variants:

```
enum E1 {
    A,
    B,
    C,
}
enum E2 {
    A(E1),
    B,
}
fn foo() {
    match E2::A(E1::A) {
        E2::A(E1::B) => {}
        E2::B => {}
    }
    //~^ ERROR `E2::A(E1::A)` and `E2::A(E1::C)` not handled
}
```

Unify look between match with no arms and match with some missing patterns.

Fix #37518.

1 files changed, 10 insertions, 2 deletions

diff --git a/src/test/ui/missing/missing-items/issue-40221.stderr b/src/test/ui/missing/missing-items/issue-40221.stderr
index 437fb7bbc31..9e4ce7c81b7 100644
--- a/src/test/ui/missing/missing-items/issue-40221.stderr
+++ b/src/test/ui/missing/missing-items/issue-40221.stderr
@@ -1,8 +1,16 @@
 error[E0004]: non-exhaustive patterns: `C(QA)` not covered
   --> $DIR/issue-40221.rs:11:11
    |
-LL |     match proto { //~ ERROR non-exhaustive patterns
-   |           ^^^^^ pattern `C(QA)` not covered
+LL | / enum P {
+LL | |     C(PC),
+   | |     - not covered
+LL | | }
+   | |_- `P` defined here
+...
+LL |       match proto { //~ ERROR non-exhaustive patterns
+   |             ^^^^^ pattern `C(QA)` not covered
+   |
+   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
 
 error: aborting due to previous error