Remove Ident::empty.

All uses have been removed. And it's nonsensical: an identifier by
definition has at least one char.

The commits adds an is-non-empty assertion to `Ident::new` to enforce
this, and converts some `Ident` constructions to use `Ident::new`.
Adding the assertion requires making `Ident::new` and
`Ident::with_dummy_span` non-const, which is no great loss.

The commit amends a couple of places that do path splitting to ensure no
empty identifiers are created.

compiler/rustc_parse/src/parser/expr.rs

@@ -3828,7 +3828,7 @@ impl<'a> Parser<'a> {
         // Convert `label` -> `'label`,
         // so that nameres doesn't complain about non-existing label
         let label = format!("'{}", ident.name);
-        let ident = Ident { name: Symbol::intern(&label), span: ident.span };
+        let ident = Ident::new(Symbol::intern(&label), ident.span);
 
         self.dcx().emit_err(errors::ExpectedLabelFoundIdent {
             span: ident.span,