CFE ignores custom toString() method in enums

[alexmarkov]

This is a part of #47453.

Test co19/src/LanguageFeatures/Enhanced-Enum/semantics_A04_t02 currently fails with RuntimeError on the VM because custom implementation of toString() method in the enum class is ignored and replaced with synthetic in the front-end.

tests/co19/src/LanguageFeatures/Enhanced-Enum/semantics_A04_t02.dart:

enum E1 {
  e1,
  e2,
  e3;

  String toString() => "Enum E1.${this.name}";
}

Kernel:

  class E1 extends core::_Enum /*isEnum*/  {
    static const field core::List<sem::E1> values = #C10;
    static const field sem::E1 e1 = #C3;
    static const field sem::E1 e2 = #C6;
    static const field sem::E1 e3 = #C9;
    const constructor •(core::int index, core::String name) → sem::E1
      : super core::_Enum::•(index, name)
      ;
    method toString() → core::String
      return "E1.${this.{core::_Enum::_name}{core::String}}";
  }

/cc @chloestefantsova @johnniwinther