[lld-macho] Support creating N_SO stab for DWARF5 compile units

In DWARF5, the `DW_AT_name` and `DW_AT_comp_dir` attributes are encoded
using the `strx*` forms, which specify an index into `__debug_str_offs`.
This commit adds that section to DwarfObject, so the debug info parser
can resolve these references.

The test case was manually adapted from stabs-icf.s.

Fixes #51668

Differential Revision: https://reviews.llvm.org/D130559