On Darwin allow for sanitizer malloc implementations to provide a zone

enumerator.

This is done by defining `COMMON_MALLOC_HAS_ZONE_ENUMERATOR` to `1` and
then by providing an implementation of the `mi_enumerator(...)` function.
If a custom implementation isn't desired the macro is set to `0` which
causes a stub version (that fails) to be used.

Currently all Darwin sanitizers that have malloc implementations define
this to be `0` so there is no functionality change.

rdar://problem/45284065

llvm-svn: 351711