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_parse/src/parser/expr.rs

@@ -724,7 +724,9 @@ impl<'a> Parser<'a> {
     /// Returns the span of expr if it was not interpolated, or the span of the interpolated token.
     fn interpolated_or_expr_span(&self, expr: &Expr) -> Span {
         match self.prev_token.kind {
-            TokenKind::Interpolated(..) => self.prev_token.span,
+            TokenKind::NtIdent(..) | TokenKind::NtLifetime(..) | TokenKind::Interpolated(..) => {
+                self.prev_token.span
+            }
             _ => expr.span,
         }
     }