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.

compiler/rustc_errors/src/diagnostic.rs

@@ -116,8 +116,6 @@ pub struct Diagnostic {
     /// `span` if there is one. Otherwise, it is `DUMMY_SP`.
     pub sort_span: Span,
 
-    /// If diagnostic is from Lint, custom hash function ignores children.
-    /// Otherwise hash is based on the all the fields.
     pub is_lint: Option<IsLint>,
 
     /// With `-Ztrack_diagnostics` enabled,
@@ -990,22 +988,24 @@ impl Diagnostic {
     ) -> (
         &Level,
         &[(DiagnosticMessage, Style)],
-        Vec<(&Cow<'static, str>, &DiagnosticArgValue<'static>)>,
         &Option<ErrCode>,
-        &Option<IsLint>,
         &MultiSpan,
+        &[SubDiagnostic],
         &Result<Vec<CodeSuggestion>, SuggestionsDisabled>,
-        Option<&[SubDiagnostic]>,
+        Vec<(&Cow<'static, str>, &DiagnosticArgValue<'static>)>,
+        &Option<IsLint>,
     ) {
         (
             &self.level,
             &self.messages,
-            self.args().collect(),
             &self.code,
-            &self.is_lint,
             &self.span,
+            &self.children,
             &self.suggestions,
-            (if self.is_lint.is_some() { None } else { Some(&self.children) }),
+            self.args().collect(),
+            // omit self.sort_span
+            &self.is_lint,
+            // omit self.emitted_at
         )
     }
 }

src/tools/clippy/tests/ui/same_name_method.stderr

@@ -61,5 +61,17 @@ note: existing `foo` defined here
 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:81:9
+   |
+LL |         impl T2 for S {}
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
 

tests/rustdoc-ui/unescaped_backticks.stderr

@@ -302,7 +302,6 @@ LL | |     /// level changes.
    = help: if you meant to use a literal backtick, escape it
             change: or `None` if it isn't.
            to this: or `None\` if it isn't.
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: unescaped backtick
   --> $DIR/unescaped_backticks.rs:323:5
@@ -322,7 +321,6 @@ LL | |     /// level changes.
    = help: if you meant to use a literal backtick, escape it
             change: `on_event` should be called.
            to this: `on_event\` should be called.
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: unescaped backtick
   --> $DIR/unescaped_backticks.rs:323:5
@@ -342,7 +340,6 @@ LL | |     /// level changes.
    = help: if you meant to use a literal backtick, escape it
             change: [`rebuild_interest_cache`][rebuild] is called after the value of the max
            to this: [`rebuild_interest_cache\`][rebuild] is called after the value of the max
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 error: unescaped backtick
   --> $DIR/unescaped_backticks.rs:349:56

tests/ui/consts/const-eval/stable-metric/ctfe-simple-loop.warn.stderr

@@ -40,7 +40,6 @@ help: the constant being evaluated
    |
 LL | const Y: u32 = simple_loop(35);
    | ^^^^^^^^^^^^
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 warning: constant evaluation is taking a long time
   --> $DIR/ctfe-simple-loop.rs:9:5

tests/ui/imports/ambiguous-9.stderr

@@ -60,7 +60,6 @@ note: `date_range` could also refer to the function imported here
 LL | use prelude::*;
    |     ^^^^^^^^^^
    = help: consider adding an explicit import of `date_range` to disambiguate
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
 
 warning: 4 warnings emitted