Emit warning when named arguments are used positionally in format

Addresses Issue 98466 by emitting a warning if a named argument
is used like a position argument (i.e. the name is not used in
the string to be formatted).

Fixes rust-lang#98466

compiler/rustc_lint_defs/src/lib.rs

@@ -467,6 +467,7 @@ pub enum BuiltinLintDiagnostics {
         /// If true, the lifetime will be fully elided.
         use_span: Option<(Span, bool)>,
     },
+    NamedArgumentUsedPositionally(Span, Span, String),
 }
 
 /// Lints that are buffered up early on in the `Session` before the