Remove excess use of auto on initializers (auto x = Y(z) -> Y x(z)) (#4239)

explorer/syntax/parse.cpp

@@ -22,7 +22,7 @@ static auto ParseImpl(yyscan_t scanner, Nonnull<Arena*> arena,
                              file_kind, parser_debug);
 
   // Do the parse.
-  auto parser = Parser(arena, scanner, context, &ast);
+  Parser parser(arena, scanner, context, &ast);
   if (parser_debug) {
     parser.set_debug_level(1);
   }