Make more allocator methods work correctly with in an out-of-process mode.

Summary:
This makes `GetBlockBegin()` and `GetBlockBeginFastLocked()` work correctly with `RemoteAddressSpaceView`.

This has a knock on effect of also making the `PointerIsMine()` and
`GetMetaData()` methods behave correctly when `RemoteAddressSpaceView`
is used to instantiate the allocators.

This will be used by future out-of-process allocator enumeration
patches.

rdar://problem/45284065

Reviewers: kcc, vitalybuka, dvyukov, cryptoad, eugenis, george.karpenkov, yln

Subscribers: #sanitizers, llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D56964

llvm-svn: 352335