Start adding the supporting code to perform out-of-process allocator

enumeration.

Summary:

This patch introduces the local portion (`LocalAddressSpaceView`) of the
`AddressSpaceView` abstraction and modifies the secondary allocator
so that the `ForEachChunk()` method (and its callees) would work in the
out-of-process case when `AddressSpaceView` is `RemoteAddressSpaceView`.

The `AddressSpaceView` abstraction simply maps pointers from a target
process to a pointer in the local process (via its `Load()` method). For
the local (in-process) case this is a no-op. For the remote
(out-of-process) case this is not a no-op. The implementation of the
out-of-process `RemoteAddressSpaceView` is not included in this patch
and will be introduced later.

This patch is considerably simpler than the `ObjectView` abstraction
used in previous patches but lacks the type safety and stricter memory
management of the `ObjectView` abstraction.

This patch does not introduce any tests because with
`LocalAddressSpaceView` it should be a non functional change and unit
tests already cover the secondary allocator.  When
`RemoteAddressSpaceView` is landed tests will be added to ensure that it
functions as expected.

rdar://problem/45284065

Reviewers: kcc, kubamracek, dvyukov, vitalybuka, cryptoad,
george.karpenkov, morehouse

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D53975

llvm-svn: 346956