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.

compiler/rustc_attr_parsing/src/context.rs

@@ -222,7 +222,7 @@ impl<'sess> AttributeParser<'sess> {
             // if we're only looking for a single attribute,
             // skip all the ones we don't care about
             if let Some(expected) = self.parse_only {
-                if attr.name_or_empty() != expected {
+                if !attr.has_name(expected) {
                     continue;
                 }
             }
@@ -232,7 +232,7 @@ impl<'sess> AttributeParser<'sess> {
             // that's expanded right? But no, sometimes, when parsing attributes on macros,
             // we already use the lowering logic and these are still there. So, when `omit_doc`
             // is set we *also* want to ignore these
-            if omit_doc == OmitDoc::Skip && attr.name_or_empty() == sym::doc {
+            if omit_doc == OmitDoc::Skip && attr.has_name(sym::doc) {
                 continue;
             }
 
@@ -250,7 +250,7 @@ impl<'sess> AttributeParser<'sess> {
                     }))
                 }
                 // // FIXME: make doc attributes go through a proper attribute parser
-                // ast::AttrKind::Normal(n) if n.name_or_empty() == sym::doc => {
+                // ast::AttrKind::Normal(n) if n.has_name(sym::doc) => {
                 //     let p = GenericMetaItemParser::from_attr(&n, self.dcx());
                 //
                 //     attributes.push(Attribute::Parsed(AttributeKind::DocComment {