Remove LangItems::require

It's just a short wrapper used by `tcx.require_lang_item`. Deleting it
gives us a negative diff.

compiler/rustc_middle/src/error.rs

@@ -1,5 +1,5 @@
 use rustc_macros::Diagnostic;
-use rustc_span::Span;
+use rustc_span::{Span, Symbol};
 
 use crate::ty::Ty;
 
@@ -73,6 +73,14 @@ pub(crate) struct StrictCoherenceNeedsNegativeCoherence {
     pub attr_span: Option<Span>,
 }
 
+#[derive(Diagnostic)]
+#[diag(middle_requires_lang_item)]
+pub(crate) struct RequiresLangItem {
+    #[primary_span]
+    pub span: Option<Span>,
+    pub name: Symbol,
+}
+
 #[derive(Diagnostic)]
 #[diag(middle_const_not_used_in_type_alias)]
 pub(super) struct ConstNotUsedTraitAlias {