Remove Nonterminal::NtTT.

It's only needed for macro expansion, not as a general element in the
AST. This commit removes it, adds `NtOrTt` for the parser and macro
expansion cases, and renames the variants in `NamedMatch` to better
match the new type.

compiler/rustc_ast/src/mut_visit.rs

@@ -787,7 +787,6 @@ pub fn visit_interpolated<T: MutVisitor>(nt: &mut token::Nonterminal, vis: &mut
             visit_lazy_tts(tokens, vis);
         }
         token::NtPath(path) => vis.visit_path(path),
-        token::NtTT(tt) => visit_tt(tt, vis),
         token::NtVis(visib) => vis.visit_vis(visib),
     }
 }