Remove NtItem and NtStmt.

This involves replacing `nt_pretty_printing_compatibility_hack` with
`stream_pretty_printing_compatibility_hack`.

The handling of statements in `transcribe` is slightly different to
other nonterminal kinds, due to the lack of `from_ast` implementation
for empty statements.

Notable test changes:
- `tests/ui/proc-macro/expand-to-derive.rs`: the diff looks large but
  the only difference is the insertion of a single invisible-delimited
  group around a metavar.

compiler/rustc_parse/src/parser/mod.rs

@@ -1076,10 +1076,12 @@ impl<'a> Parser<'a> {
         let initial_semicolon = self.token.span;
 
         while self.eat(exp!(Semi)) {
-            let _ = self.parse_stmt_without_recovery(false, ForceCollect::No).unwrap_or_else(|e| {
-                e.cancel();
-                None
-            });
+            let _ = self
+                .parse_stmt_without_recovery(false, ForceCollect::No, false)
+                .unwrap_or_else(|e| {
+                    e.cancel();
+                    None
+                });
         }
 
         expect_err
@@ -1746,6 +1748,8 @@ pub enum ParseNtResult {
     Tt(TokenTree),
     Ident(Ident, IdentIsRaw),
     Lifetime(Ident, IdentIsRaw),
+    Item(P<ast::Item>),
+    Stmt(P<ast::Stmt>),
     Pat(P<ast::Pat>, NtPatKind),
     Ty(P<ast::Ty>),
     Meta(P<ast::AttrItem>),