rustc_span: Ident::invalid -> Ident::empty

The equivalent for `Symbol`s was renamed some time ago (`kw::Invalid` -> `kw::Empty`), and it makes sense to do the same thing for `Ident`s.

compiler/rustc_span/src/symbol.rs

@@ -1453,7 +1453,7 @@ impl Ident {
     }
 
     #[inline]
-    pub fn invalid() -> Ident {
+    pub fn empty() -> Ident {
         Ident::with_dummy_span(kw::Empty)
     }