Add new `__sanitizer_mz_default_zone()` API which returns the address of the...

Add new `__sanitizer_mz_default_zone()` API which returns the address of the ASan malloc zone. This API will be used for testing in future patches.

Summary:
The name of the function is based on `malloc_default_zone()` found
in Darwin's `malloc/malloc.h` header file.

Reviewers: kubamracek, george.karpenkov

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D55064

llvm-svn: 348481