Rollup merge of #80495 - jyn514:rename-empty, r=petrochenkov

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.

r? `@petrochenkov`

compiler/rustc_resolve/src/late.rs

@@ -1639,7 +1639,7 @@ impl<'a: 'ast, 'b, 'ast> LateResolutionVisitor<'a, 'b, 'ast> {
         }
 
         // Record as bound if it's valid:
-        let ident_valid = ident.name != kw::Invalid;
+        let ident_valid = ident.name != kw::Empty;
         if ident_valid {
             bindings.last_mut().unwrap().1.insert(ident);
         }