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_parse/src/lib.rs

@@ -289,7 +289,6 @@ pub fn nt_to_tokenstream(
         Nonterminal::NtMeta(ref attr) => convert_tokens(attr.tokens.as_ref()),
         Nonterminal::NtPath(ref path) => convert_tokens(path.tokens.as_ref()),
         Nonterminal::NtVis(ref vis) => convert_tokens(vis.tokens.as_ref()),
-        Nonterminal::NtTT(ref tt) => Some(tt.clone().into()),
         Nonterminal::NtExpr(ref expr) | Nonterminal::NtLiteral(ref expr) => {
             prepend_attrs(&expr.attrs, expr.tokens.as_ref())
         }