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_lowering/src/lib.rs

@@ -1716,7 +1716,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         }
         self.arena.alloc_from_iter(inputs.iter().map(|param| match param.pat.kind {
             PatKind::Ident(_, ident, _) => ident,
-            _ => Ident::new(kw::Invalid, param.pat.span),
+            _ => Ident::new(kw::Empty, param.pat.span),
         }))
     }