[Patch] reduce template instantiation depth in <variant>

[Barrett Adair]

Hi,

Currently, std::variant exceeds the default ftemplate-depth parameter
when instantiated with 297 types. This small patch increases this
ceiling to 446 types (from the bottom of the template stack).

This was accomplished by using fold expressions instead of the __and_ template.

Thanks,
Barrett
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< 		  __and_<std::is_trivially_destructible<_Types>...>::value>
---
> 		  (std::is_trivially_destructible<_Types>::value &&...)>
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<       noexcept(__and_<is_nothrow_move_constructible<_Types>...>::value)
---
>       noexcept((is_nothrow_move_constructible<_Types>::value && ...))
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<       noexcept(__and_<is_nothrow_move_constructible<_Types>...,
< 		      is_nothrow_move_assignable<_Types>...>::value)
---
>       noexcept((is_nothrow_move_constructible<_Types>::value && ...)
> 		      && (is_nothrow_move_assignable<_Types>::value && ...))
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< 	__and_<is_copy_constructible<_Types>...>::value,
< 	__and_<is_copy_constructible<_Types>...,
< 	       is_move_constructible<_Types>...,
< 	       is_copy_assignable<_Types>...>::value,
< 	__and_<is_move_constructible<_Types>...>::value,
< 	__and_<is_move_constructible<_Types>...,
< 	       is_move_assignable<_Types>...>::value,
---
>   (is_copy_constructible_v<_Types> && ...),
>   (is_copy_constructible_v<_Types> && ...)
>       && (is_move_constructible_v<_Types> && ...)
>       && (is_copy_assignable_v<_Types> && ...),
> 	(is_move_constructible_v<_Types> && ...),
> 	(is_move_constructible_v<_Types> && ...)
>       && (is_move_assignable_v<_Types> && ...),
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<       noexcept(__and_<
< 	is_nothrow_move_constructible<_Types>...>::value) = default;
---
>       noexcept((is_nothrow_move_constructible<_Types>::value &&...)) = default;
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< 	       typename = enable_if_t<__and_<__is_uses_allocator_constructible<
---
> 	       typename = enable_if_t<(__is_uses_allocator_constructible<
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< 		 add_lvalue_reference_t<add_const_t<_Types>>>...>::value>>
---
> 		 add_lvalue_reference_t<add_const_t<_Types>>>::value && ...)>>
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< 	       typename = enable_if_t<__and_<
---
> 	       typename = enable_if_t<(
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< 		   _Types, _Alloc, add_rvalue_reference_t<_Types>>...>::value>>
---
> 		   _Types, _Alloc, add_rvalue_reference_t<_Types>>::value && ...)>>
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<       noexcept(__and_<is_nothrow_move_constructible<_Types>...,
< 		      is_nothrow_move_assignable<_Types>...>::value) = default;
---
>       noexcept((is_nothrow_move_constructible<_Types>::value && ...)
> 		      && (is_nothrow_move_assignable<_Types>::value && ...)) = default;
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<       noexcept(__and_<__is_nothrow_swappable<_Types>...>::value
---
>       noexcept((__is_nothrow_swappable<_Types>::value && ...)