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_hir/src/hir/tests.rs

@@ -50,21 +50,14 @@ fn trait_object_roundtrips() {
 }
 
 fn trait_object_roundtrips_impl(syntax: TraitObjectSyntax) {
-    let unambig = TyKind::TraitObject::<'_, ()>(
-        &[],
-        TaggedRef::new(
-            &const {
-                Lifetime {
-                    hir_id: HirId::INVALID,
-                    ident: Ident::new(sym::name, DUMMY_SP),
-                    kind: LifetimeKind::Static,
-                    source: LifetimeSource::Other,
-                    syntax: LifetimeSyntax::Hidden,
-                }
-            },
-            syntax,
-        ),
-    );
+    let lt = Lifetime {
+        hir_id: HirId::INVALID,
+        ident: Ident::new(sym::name, DUMMY_SP),
+        kind: LifetimeKind::Static,
+        source: LifetimeSource::Other,
+        syntax: LifetimeSyntax::Hidden,
+    };
+    let unambig = TyKind::TraitObject::<'_, ()>(&[], TaggedRef::new(&lt, syntax));
     let unambig_to_ambig = unsafe { std::mem::transmute::<_, TyKind<'_, AmbigArg>>(unambig) };
 
     match unambig_to_ambig {