Introduce a way for Languages to specify whether values of "reference types"...

Introduce a way for Languages to specify whether values of "reference types" are "nil" (not pointing to anything) or uninitialized (never made to point at anything)
This latter determination may or may not be possible on a per-language basis; and neither is mandatory to implement for any language

Use this knowledge in the ValueObjectPrinter to generalize the notion of IsObjCNil() and the respective printout

llvm-svn: 252663