Change TokenKind's Print overload to a format_provider. (#2534)

Fundamentally this `.Print()` is wrong for debug output at present because `.fixed_spelling()` can be empty. It's also inconsistent with other enums to use it. We frequently print tokens for debugging, and it's easy to forget to specify `.name()` there.

Diagnostics use formatv, so we can provide a format_provider and address it in one spot that way. It also makes it harder to just forget to do the right thing.

toolchain/parser/parse_tree.cpp

@@ -95,7 +95,7 @@ auto ParseTree::PrintNode(llvm::raw_ostream& output, Node n, int depth,
   if (preorder) {
     output << "node_index: " << n << ", ";
   }
-  output << "kind: '" << n_impl.kind.name() << "', text: '"
+  output << "kind: '" << n_impl.kind << "', text: '"
          << tokens_->GetTokenText(n_impl.token) << "'";
 
   if (n_impl.has_error) {