Replace infallible `name_or_empty` methods with fallible `name` methods.

I'm removing empty identifiers everywhere, because in practice they
always mean "no identifier" rather than "empty identifier". (An empty
identifier is impossible.) It's better to use `Option` to mean "no
identifier" because you then can't forget about the "no identifier"
possibility.

Some specifics:
- When testing an attribute for a single name, the commit uses the
  `has_name` method.
- When testing an attribute for multiple names, the commit uses the new
  `has_any_name` method.
- When using `match` on an attribute, the match arms now have `Some` on
  them.

In the tests, we now avoid printing empty identifiers by not printing
the identifier in the `error:` line at all, instead letting the carets
point out the problem.

1 files changed, 2 insertions, 3 deletions

diff --git a/compiler/rustc_incremental/src/errors.rs b/compiler/rustc_incremental/src/errors.rs
index b4a207386dc..dbc72d085be 100644
--- a/compiler/rustc_incremental/src/errors.rs
+++ b/compiler/rustc_incremental/src/errors.rs
@@ -107,11 +107,10 @@ pub(crate) struct NotLoaded<'a> {
 }
 
 #[derive(Diagnostic)]
-#[diag(incremental_unknown_item)]
-pub(crate) struct UnknownItem {
+#[diag(incremental_unknown_rustc_clean_argument)]
+pub(crate) struct UnknownRustcCleanArgument {
     #[primary_span]
     pub span: Span,
-    pub name: Symbol,
 }
 
 #[derive(Diagnostic)]