Debug Info: Support address space attributes on rvalue references.

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.

rdar://problem/50511483

Differential Revision: https://reviews.llvm.org/D61625

llvm-svn: 360176