LWG 2843 "Unclear behavior of std::pmr::memory_resource::do_allocate()"

Patch by Arthur O'Dwyer.
Reviewed as https://reviews.llvm.org/D47344

new_delete_resource().allocate(n, a) has basically two permissible results:

* Return an appropriately sized and aligned block.
* Throw bad_alloc.

Before this patch, libc++'s new_delete_resource would do a third and impermissible thing, which was
to return an appropriately sized but inappropriately under-aligned block. This is now fixed.

(This came up while I was stress-testing unsynchronized_pool_resource on my MacBook. If we can't
trust the default resource to return appropriately aligned blocks, pretty much everything breaks.
For similar reasons, I would strongly support just patching __libcpp_allocate directly, but I don't
care to die on that hill, so I made this patch as a <memory_resource>-specific workaround.)

llvm-svn: 355763