Remove a Span from TokenKind::Interpolated.

This span records the declaration of the metavariable in the LHS of the macro.
It's used in a couple of error messages. Unfortunately, it gets in the way of
the long-term goal of removing `TokenKind::Interpolated`. So this commit
removes it, which degrades a couple of (obscure) error messages but makes
things simpler and enables the next commit.

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -926,7 +926,7 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
             }
             token::Eof => "<eof>".into(),
 
-            token::Interpolated(ref nt) => self.nonterminal_to_string(&nt.0).into(),
+            token::Interpolated(ref nt) => self.nonterminal_to_string(&nt).into(),
         }
     }