Features: Add cxx_delegating_constructors.
Features: Add cxx_variadic_templates.

Expect cxx_variadic_templates to implement N2555.

N2555 is essentially a bugfix and predates most compiler releases which
aimed to experimentally support variadic templates.
Features: Add cxx_constexpr.
Features: Add cxx_static_assert.
Features: Add cxx_final.
Features: Add cxx_override.
Features: Add cxx_auto_function.
Features: Add cxx_strong_enums.
Features: Add cxx_decltype.
Features: Add cxx_defaulted_functions.
Features: Add cxx_deleted_functions.
Features: Add cxx_explicit_conversions.
Features: Add cxx_inheriting_constructors.
Features: Add cxx_lambdas.
Features: Add cxx_nonstatic_member_init.
Features: Add cxx_noexcept.
Features: Add cxx_nullptr.
Features: Add cxx_range_for.
Features: Add cxx_raw_string_literals.
Features: Add cxx_reference_qualified_functions.