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
503f2ee4a8