[ASan] Use dylib interposition to hook memory allocation in the dynamic runtime.

This CL drastically simplifies the way we're hooking the memory allocation routines in ASan on Mac by using dylib interposition to replace the main malloc_zone_* functions. This allows us to avoid replacing the default CFAllocator and drop the CF dependency at all.

Committing this patch will result in the static runtime being broken. A follow-up CL will switch ASan to use the dynamic runtime library.

llvm-svn: 173134