PR libstdc++/90397 fix std::variant friend declarations

PR libstdc++/90397 fix std::variant friend declarations

Clang diagnoses the inconsistent noexcept-specifier on the friend
declaration of __get. Add it, and also on __get_storage.

	PR libstdc++/90397
	* include/std/variant (_Variant_storage<false, Types...>::_M_storage())
	(_Variant_storage<true, Types...>::_M_reset()))
	(_Variant_storage<true, Types...>::_M_storage())): Add noexcept.
	(__get_storage): Likewise.
	(variant): Add noexcept to friend declarations for __get and
	__get_storage.

From-SVN: r271079