Change the ABI class to have a weak pointer to its Process;

some methods in the ABI need a Process to do their work.
Instead of passing it in as a one-off argument to those
methods, this patch puts it in the base class and the methods
can retrieve if it needed.

Note that ABI's are sometimes built without a Process 
(e.g. SBTarget::GetStackRedZoneSize) so it's entirely
possible that the process weak pointer will not be
able to reconsistitue into a strong pointer.

<rdar://problem/32526754> 

llvm-svn: 306633