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
7a4be38049