Move existing diagnostic struct to a new errors module

compiler/rustc_interface/src/errors.rs

@@ -0,0 +1,19 @@
+use rustc_macros::SessionDiagnostic;
+use rustc_span::{Span, Symbol};
+
+#[derive(SessionDiagnostic)]
+#[diag(interface::ferris_identifier)]
+pub struct FerrisIdentifier {
+    #[primary_span]
+    pub spans: Vec<Span>,
+    #[suggestion(code = "ferris", applicability = "maybe-incorrect")]
+    pub first_span: Span,
+}
+
+#[derive(SessionDiagnostic)]
+#[diag(interface::emoji_identifier)]
+pub struct EmojiIdentifier {
+    #[primary_span]
+    pub spans: Vec<Span>,
+    pub ident: Symbol,
+}