Add support for sythetic operator dereference

Add support for sythetic operator dereference

Summary:
After this change a sythetic child provider can generate a special child
named "$$dereference$$" what if present is used when "operator*" or
"operator->" used on a ValueObject. The goal of the change is to make
expressions like "up->foo" work inside the "frame variable" command.

Reviewers: labath, jingham

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D31368

llvm-svn: 299251