Fixed version of https://reviews.llvm.org/D38437 (fixes Win/Fuchsia failures).
Creating a new revision, since the old one was getting a bit old/crowded.
In Fuchsia, MmapNoAccess/MmapFixedOrDie are implemented using a global VMAR, which means that MmapNoAccess can only be called once. This works for the sanitizer allocator but *not* for the Scudo allocator. Hence, this changeset introduces a new ReservedAddressRange object to serve as the new API for these calls. In this changeset, the object still calls into the old Mmap implementations. The next changeset two changesets will convert the sanitizer and scudo allocators to use the new APIs, respectively. (ReservedAddressRange will replace the SecondaryHeader in Scudo.) Finally, a last changeset will update the Fuchsia implementation.