Remove NtMeta.

Note: there was an existing code path involving `Interpolated` in
`MetaItem::from_tokens` that was dead. This commit transfers that to the
new form, but puts an `unreachable!` call inside it.

compiler/rustc_parse/src/parser/mod.rs

@@ -1748,6 +1748,7 @@ pub enum ParseNtResult {
     Lifetime(Ident, IdentIsRaw),
     Pat(P<ast::Pat>, NtPatKind),
     Ty(P<ast::Ty>),
+    Meta(P<ast::AttrItem>),
     Vis(P<ast::Visibility>),
 
     /// This variant will eventually be removed, along with `Token::Interpolate`.