This effectively reverts SVN r204290 and r204292 (back when this code was part of libcxxabi).
According to SVN r204290, the primary architecture using the DW_CFA_GNU_args_size opcode is VAX.
However, clang also produces it on X86 when it has done X86CallFrameOptimization, which gets done much more frequently if the stack is aligned to 4 bytes (which is the default when targeting windows).
This issue can be tested by building code for x86 linux (at least for 32 bit) with clang with -mstack-alignment=4.
I'm not sure if this code should be handled differently for VAX with some ifdef, or what the correct interpretation of this opcode is. I ran into this issue while trying to use libunwind for 32 bit windows (where clang uses a 4 byte stack alignment by default), found this commit, and noticed I got it working by reverting it. And later noticed I could reproduce the same issue on 32 bit x86 linux as well, by forcing -mstack-alignment=4.