[lldb][DWARFASTParserClang] Don't create unnamed bitfields to account for vtable pointer

**Summary**

When filling out the LayoutInfo for a structure with the offsets
from DWARF, LLDB fills gaps in the layout by creating unnamed
bitfields and adding them to the AST. If we don't do this correctly
and our layout has overlapping fields, we will hat an assertion
in `clang::CGRecordLowering::lower()`. Specifically, if we have
a derived class with a VTable and a bitfield immediately following
the vtable pointer, we create a layout with overlapping fields.

This is an oversight in some of the previous cleanups done around this
area.

In `D76808`, we prevented LLDB from creating unnamed bitfields if there
was a gap between the last field of a base class and the start of a bitfield
in the derived class.

In `D112697`, we started accounting for the vtable pointer. The intention
there was to make sure the offset bookkeeping accounted for the
existence of a vtable pointer (but we didn't actually want to create
any AS...