The __pool_resource::_M_unpooled member was declared with type
std::vector, which means that the type depends on whether debug mode is
active or not. Because the non-inline definitions in
src/c++17/memory_resource.cc are never compiled with debug mode, the
type declared in the header doesn't match the type in the library
definitions, leading to undefined behaviour.

The solution is to ensure the header always uses the non-debug vector,
even when debug mode is active. To make this easier a new alias template
is defined: _GLIBCXX_STD_C::pmr::vector.

	* include/std/memory_resource (__pool_resource::_M_unpooled): Use
	normal mode vector, even for debug mode.
	* include/std/vector [_GLIBCXX_DEBUG] (_GLIBCXX_STD_C::pmr::vector):
	Define alias template for normal mode vector.

From-SVN: r268354