Add a class ValueObjectConstResultCast.

Summary:
Other changes around the main change include:

1. Add a method Cast to ValueObjectConstResult, ValueObjectConstResultImpl
and ValueObjectConstResultChild.

2. Add an argument |live_address| of type lldb::addr_t to the constructor
of ValueObjectConstResultChild. This is passed on to the backing
ValueObjectConstResultImpl object constructor so that the address of the
child value can be calculated properly.

Reviewers: granata.enrico, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D11203

llvm-svn: 242374