Make __resource_adaptor_imp usable with C++17 memory_resource

Make __resource_adaptor_imp usable with C++17 memory_resource

By making the memory_resource base class a template parameter the
__resource_adaptor_imp can be used to adapt an allocator into a
std::pmr::memory_resource instead of experimental::pmr::memory_resource.

	* include/experimental/memory_resource: Adjust comments and
	whitespace.
	(__resource_adaptor_imp): Add second template parameter for type of
	memory resource base class.
	(memory_resource): Define default constructor, destructor, copy
	constructor and copy assignment operator as defaulted.

From-SVN: r262944