<rdar://problem/15001220>

Fixed an issue with the lldb/test/lang/cpp/virtual test case had a virtual class that had a DW_TAG_inheritance child that was virtual and had a DW_AT_data_member_location of:

DW_AT_data_member_location( DW_OP_dup, DW_OP_deref, DW_OP_constu(0x00000018), DW_OP_minus, DW_OP_deref, DW_OP_plus )

We failed to evaluate this and then we were passing the incorrect offset back to clang and clang would crash. The AST external source has a function named LayoutRecordType which allows us to supply the virtual base class offsets, but that really doesn't make sense to do as clang will lay them out correctly. So we must ignore virtual base classes when doing layout.