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/ast_traits.rs

@@ -240,7 +240,6 @@ impl HasTokens for Nonterminal {
             Nonterminal::NtPath(path) => path.tokens(),
             Nonterminal::NtVis(vis) => vis.tokens(),
             Nonterminal::NtBlock(block) => block.tokens(),
-            Nonterminal::NtIdent(..) | Nonterminal::NtLifetime(..) => None,
         }
     }
     fn tokens_mut(&mut self) -> Option<&mut Option<LazyAttrTokenStream>> {
@@ -254,7 +253,6 @@ impl HasTokens for Nonterminal {
             Nonterminal::NtPath(path) => path.tokens_mut(),
             Nonterminal::NtVis(vis) => vis.tokens_mut(),
             Nonterminal::NtBlock(block) => block.tokens_mut(),
-            Nonterminal::NtIdent(..) | Nonterminal::NtLifetime(..) => None,
         }
     }
 }