Remove NtIdent and NtLifetime.

The extra span is now recorded in the new `TokenKind::NtIdent` and
`TokenKind::NtLifetime`. These both consist of a single token, and so
there's no operator precedence problems with inserting them directly
into the token stream.

The other way to do this would be to wrap the ident/lifetime in invisible
delimiters, but there's a lot of code that assumes an interpolated
ident/lifetime fits in a single token, and changing all that code to work with
invisible delimiters would have been a pain. (Maybe it could be done in a
follow-up.)

This change might not seem like much of a win, but it's a first step toward the
much bigger and long-desired removal of `Nonterminal` and
`TokenKind::Interpolated`. That change is big and complex enough that it's
worth doing this piece separately. (Indeed, this commit is based on part of a
late commit in #114647, a prior attempt at that big and complex change.)

compiler/rustc_ast/src/mut_visit.rs

@@ -781,6 +781,12 @@ pub fn visit_token<T: MutVisitor>(t: &mut Token, vis: &mut T) {
             *span = ident.span;
             return; // Avoid visiting the span for the second time.
         }
+        token::NtIdent(ident, _is_raw) => {
+            vis.visit_ident(ident);
+        }
+        token::NtLifetime(ident) => {
+            vis.visit_ident(ident);
+        }
         token::Interpolated(nt) => {
             let nt = Lrc::make_mut(nt);
             visit_nonterminal(nt, vis);
@@ -832,8 +838,6 @@ fn visit_nonterminal<T: MutVisitor>(nt: &mut token::Nonterminal, vis: &mut T) {
         token::NtPat(pat) => vis.visit_pat(pat),
         token::NtExpr(expr) => vis.visit_expr(expr),
         token::NtTy(ty) => vis.visit_ty(ty),
-        token::NtIdent(ident, _is_raw) => vis.visit_ident(ident),
-        token::NtLifetime(ident) => vis.visit_ident(ident),
         token::NtLiteral(expr) => vis.visit_expr(expr),
         token::NtMeta(item) => {
             let AttrItem { path, args, tokens } = item.deref_mut();