DWARF5, 2.12 20ff says that
Any debugging information entry representing a pointer or reference type [may have a DW_AT_address_class attribute]
The existing code (https://reviews.llvm.org/D29670) seems to take a quite literal interpretation of that wording. I don't see a reason why an rvalue reference isn't a reference type in the spirit of that paragraph. This patch allows rvalue references to also have address spaces.
Let me know if I'm missing something.