Don't hash lints differently to non-lints.

`Diagnostic::keys`, which is used for hashing and equating diagnostics,
has a surprising behaviour: it ignores children, but only for lints.
This was added in rust-lang#88493 to fix some duplicated diagnostics, but it
doesn't seem necessary any more.

This commit removes the special case and only four tests have changed
output, with additional errors. And those additional errors aren't
exact duplicates, they're just similar. For example, in
src/tools/clippy/tests/ui/same_name_method.rs we currently have this
error:
```
error: method's name is the same as an existing method in a trait
  --> $DIR/same_name_method.rs:75:13
   |
LL |             fn foo() {}
   |             ^^^^^^^^^^^
   |
note: existing `foo` defined here
  --> $DIR/same_name_method.rs:79:9
   |
LL |         impl T1 for S {}
   |         ^^^^^^^^^^^^^^^^
```
and with this change we also get this error:
```
error: method's name is the same as an existing method in a trait
  --> $DIR/same_name_method.rs:75:13
   |
LL |             fn foo() {}
   |             ^^^^^^^^^^^
   |
note: existing `foo` defined here
  --> $DIR/same_name_method.rs:81:9
   |
LL |         impl T2 for S {}
   |         ^^^^^^^^^^^^^^^^
```
I think printing this second argument is reasonable, possibly even
preferable to hiding it. And the other cases are similar.

tests/ui/same_name_method.rs

@@ -74,6 +74,7 @@ mod should_lint {
         impl S {
             fn foo() {}
             //~^ ERROR: method's name is the same as an existing method in a trait
+            //~| ERROR: method's name is the same as an existing method in a trait
         }
 
         impl T1 for S {}

tests/ui/same_name_method.stderr

@@ -56,10 +56,22 @@ LL |             fn foo() {}
    |             ^^^^^^^^^^^
    |
 note: existing `foo` defined here
-  --> $DIR/same_name_method.rs:79:9
+  --> $DIR/same_name_method.rs:80:9
    |
 LL |         impl T1 for S {}
    |         ^^^^^^^^^^^^^^^^
 
-error: aborting due to 5 previous errors
+error: method's name is the same as an existing method in a trait
+  --> $DIR/same_name_method.rs:75:13
+   |
+LL |             fn foo() {}
+   |             ^^^^^^^^^^^
+   |
+note: existing `foo` defined here
+  --> $DIR/same_name_method.rs:82:9
+   |
+LL |         impl T2 for S {}
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors