This is a preparatory change for allowing LLVM to emit DW_OP_convert
operations converting to the generic type.
If DW_OP_convert's operand is 0, it converts the top of the stack to the
generic type, as specified by DWARFv5 section 220.127.116.11:
"[...] takes one operand, which is an unsigned LEB128 integer that
represents the offset of a debugging information entry in the current
compilation unit, or value 0 which represents the generic type."
This adds support for such operations to dsymutil.
I have no knowledge of dsymutil, so I have been guided in the dark by
completely by D58534.