[dsymutil] Account for DW_OP_convert being CU relative

[dsymutil] Account for DW_OP_convert being CU relative

Currently, dsymutil treats the DW_OP_convert operand as absolute instead
of CU relative, as described by in the DWARFv5 spec, 2.5.1.6:

  "[DW_OP_convert] takes one operand, which is an unsigned LEB128 integer
  that represents the offset of a debugging information entry in the current
  compilation unit"

This patch makes dsymutil correctly treat the offset as CU relative,
preventing a crash when there are multiple compilation units.

Big thanks to Akira Hatanaka for figuring out this issue and providing
both a reduced test case and a proposed fix.