[lldb/Dataformatter] Add support to CF{Dictionary,Set}Ref types

This patch improves data formatting for CFDictionaryRef and CFSetRef.
It uses the same data-formatter as NSCFDictionaries and NSCFSets introduced
previously but did require some adjustments in Core::ValueObject.

Since the "Ref" types are opaque pointers to the actual CF containers, if the
value object has a synthetic value, lldb will use the opaque pointer's pointee
type to create the new ValueObjectChild needed to dereference the ValueObject.
This allows the "Ref" types to behaves the same as CF containers when used with
the `frame variable` command, the SBAPI or in Xcode's variable inspector.

This patch also adds support for incomplete types in ValueObject.

rdar://53104287

Differential Revision: https://reviews.llvm.org/D79554



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>