Rename kw::Invalid -> kw::Empty

See https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Is.20there.20a.20symbol.20for.20the.20empty.20string.3F/near/220054471
for context.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -2787,7 +2787,7 @@ impl<'a> State<'a> {
                     self.print_explicit_self(&eself);
                 } else {
                     let invalid = if let PatKind::Ident(_, ident, _) = input.pat.kind {
-                        ident.name == kw::Invalid
+                        ident.name == kw::Empty
                     } else {
                         false
                     };