Make sure to scan mmap'd memory regions for root pointers on OS X

Summary:
In the general case, we only need to check for root regions inside
the memory map returned by procmaps. However, on Darwin,
we also need to check inside mmap'd regions, which aren't returned
in the list of modules we get from procmaps.

This patch refactors memory region scanning on darwin to reduce
code duplication with the kernel alloc once page scan.

Reviewers: kubamracek, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32190

llvm-svn: 300760