[libc++] Make sure we use the libdispatch backend on Apple platforms

[libc++] Make sure we use the libdispatch backend on Apple platforms

The Apple.cmake cache wasn't set up properly, so we wouldn't enable
the libdispatch backend by default on Apple platforms. This patch
fixes the issue and adds a test.

We also need to make various drive-by fixes:
- Drop the usage of std::vector in libdispatch.h to avoid changing
  the transitive includes only on Apple platforms.
- Fix includes
- Use __construct at since construct_at is unavailable in C++17
- Get rid of the (unused) __get_memory_resource function since that
  adds a back-deployment requirement and we don't use it right now.
- Fix bugs in the chunking logic around boundary conditions.

Differential Revision: https://reviews.llvm.org/D155649