[LLDB][NativePDB] Fix the case when S_DEFRANGE_SUBFIELD_REGISTERs are out of order.

Previously, I was assuming that S_DEFRANGE_SUBFIELD_REGISTERs are always in the
increasing order of offset_in_parent until I saw a counter example.

Using `std::map` so that they are sorted by offset_in_parent.

Differential Revision: https://reviews.llvm.org/D124061