It was pointed out to me that an offset of 0 makes sense for ObjC, but not always for C++, and this API claims to be general enough that it should not drop C++ usability on the floor for no good reason. Fix it with an explicit offset argument

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@216487 91177308-0d34-0410-b5e6-96231b3b80d8