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_middle/src/hir/map/mod.rs

@@ -379,7 +379,7 @@ impl<'hir> Map<'hir> {
     pub fn body_param_names(&self, id: BodyId) -> impl Iterator<Item = Ident> + 'hir {
         self.body(id).params.iter().map(|arg| match arg.pat.kind {
             PatKind::Binding(_, _, ident, _) => ident,
-            _ => Ident::new(kw::Invalid, rustc_span::DUMMY_SP),
+            _ => Ident::new(kw::Empty, rustc_span::DUMMY_SP),
         })
     }