Introduce `sanitizer_malloc_introspect_t` for Darwin which is a sub-class of...

Introduce `sanitizer_malloc_introspect_t` for Darwin which is a sub-class of Darwin's `malloc_introspection_t` and use it when setting up the malloc zone.

Summary:
Currently `sanitizer_malloc_introspection_t` just adds a version field
which is used to version the allocator ABI. The current allocator ABI
version is returned by the new `GetMallocZoneAllocatorEnumerationVersion()` function.

The motivation behind this change is to allow external processes to
determine the allocator ABI of a sanitized process.

rdar://problem/45284065

Reviewers: kubamracek, george.karpenkov, vitalybuka

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D54045

llvm-svn: 346420