Fix the configuration of the Primary allocator for Darwin ARM64 by

changing the value of `SANITIZER_MMAP_RANGE_SIZE` to something more
sensible. The available VMA is at most 64GiB and not 256TiB that
was previously being used.

This change gives us several wins:

* Drastically improves LeakSanitizer performance on
  Darwin ARM64 devices. On a simple synthentic benchmark
  this took leak detection time from ~30 seconds to 0.5 seconds
  due to the `ForEachChunk(...)` method enumerating a much smaller
  number of regions. Previously we would pointlessly iterate
  over a large portion of the SizeClassAllocator32's ByteMap
  that would could never be set due it being configured for a much
  larger VM space than is actually availble.

* Decreases the memory required for the Primary allocator.
  Previously the ByteMap inside the the allocator used
  an array of pointers that took 512KiB of space. Now the required
  space for the array is 128 bytes.

rdar://problem/43509428

Differential Revision: https://reviews.llvm.org/D51173

llvm-svn: 342868