[Core] Use the implementation method GetAddressOf in ValueObjectConstResultChild

Summary:
This patch allows to retrieve an address object for `ValueObject`'s children
retrieved through e.g. `GetChildAtIndex` or `GetChildMemberWithName`. It just
uses the corresponding method of the implementation object `m_impl` to achieve
that.

Reviewers: zturner, JDevlieghere, clayborg, labath, serge-sans-paille

Reviewed By: clayborg

Subscribers: leonid.mashinskiy, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D56147

llvm-svn: 351065