On Darwin add allocator address and size fields to

`sanitizer_malloc_introspection_t` and initialize them to zero.

We allow sanitizer implementations to perform different initialization
by defining `COMMON_MALLOC_HAS_EXTRA_INTROSPECTION_INIT` to be `1`
and providing an implementation of `mi_extra_init(...)`.

We use these changes in future patches to implement malloc zone enumeration.

rdar://problem/45284065

llvm-svn: 351712