PR libstdc++/87544 limit max_size() to PTRDIFF_MAX / sizeof(T)

PR libstdc++/87544 limit max_size() to PTRDIFF_MAX / sizeof(T)

The C++17 standard requires the default implementation for
allocator_traits::max_size to return SIZE_MAX / sizeof(value_type).
That causes GCC to warn because the value could be larger than can
sensibly be passed to malloc. This patch changes the new_allocator and
malloc_allocator max_size() members to use PTRDIFF_MAX instead of
SIZE_MAX (and because they define it, the allocator_traits default isn't
used). This also changes vector::max_size to impose a sensible limit
using PTRDIFF_MAX for cases where the value from the allocator or
allocator_traits is not sensible.

	PR libstdc++/87544
	* include/bits/stl_vector.h (vector::_S_max_size): Limit size to
	PTRDIFF_MAX / sizeof(value_type).
	* include/ext/malloc_allocator.h (malloc_allocator::max_size):
	Likewise.
	* include/ext/new_allocator.h (new_allocator::max_size): Likewise.
	* testsuite/23_containers/vector/allocator/minimal.cc: Adjust
	expected value for max_size().
	* testsuite/23_containers/vector/capacity/87544.cc: New test.

From-SVN: r265021