Correctly handle stability of #[diagnostic] attributes

This commit changes the way we treat the stability of attributes in the
`#[diagnostic]` namespace. Instead of relaying on ad-hoc checks to
ensure at call side that a certain attribute is really usable at that
location it centralises the logic to one place. For diagnostic
attributes comming from other crates it just skips serializing
attributes that are not stable and that do not have the corresponding
feature enabled. For attributes from the current crate we can just use
the feature information provided by `TyCtx`.

compiler/rustc_feature/src/lib.rs

@@ -130,8 +130,9 @@ pub fn find_feature_issue(feature: Symbol, issue: GateIssue) -> Option<NonZero<u
 pub use accepted::ACCEPTED_FEATURES;
 pub use builtin_attrs::{
     deprecated_attributes, encode_cross_crate, find_gated_cfg, is_builtin_attr_name,
-    is_valid_for_get_attr, AttributeDuplicates, AttributeGate, AttributeSafety, AttributeTemplate,
-    AttributeType, BuiltinAttribute, GatedCfg, BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
+    is_stable_diagnostic_attribute, is_valid_for_get_attr, AttributeDuplicates, AttributeGate,
+    AttributeSafety, AttributeTemplate, AttributeType, BuiltinAttribute, GatedCfg,
+    BUILTIN_ATTRIBUTES, BUILTIN_ATTRIBUTE_MAP,
 };
 pub use removed::REMOVED_FEATURES;
 pub use unstable::{Features, INCOMPATIBLE_FEATURES, UNSTABLE_FEATURES};