Fix PR37694 - std::vector doesn't correctly move construct allocators.

C++2a[container.requirements.general]p8 states that when move constructing
a container, the allocator is move constructed. Vector previously copy
constructed these allocators. This patch fixes that bug.

Additionally it cleans up some unnecessary allocator conversions
when copy constructing containers. Libc++ uses
__internal_allocator_traits::select_on_copy_construction to select
the correct allocator during copy construction, but it unnecessarily
converted the resulting allocator to the user specified allocator
type and back. After this patch list and forward_list no longer
do that.

Technically we're supposed to be using allocator_traits<allocator_type>::select_on_copy_construction,
but that should seemingly be addressed as a separate patch, if at all.

llvm-svn: 334053