Lint against more useless #[must_use] attributes

This expands the existing `#[must_use]` check in `unused_attributes`
to lint against pretty much everything `#[must_use]` doesn't support.
Fixes #93906.

compiler/rustc_macros/src/session_diagnostic.rs

@@ -587,7 +587,6 @@ impl<'a> SessionDiagnosticDeriveBuilder<'a> {
         // next call to `it.next()` retrieves the next character.
         while let Some(c) = it.next() {
             if c == '{' && *it.peek().unwrap_or(&'\0') != '{' {
-                #[must_use]
                 let mut eat_argument = || -> Option<String> {
                     let mut result = String::new();
                     // Format specifiers look like