[lld-macho][nfc] Simplify LC_DATA_IN_CODE generation

1. After D113241, we have the section address easily accessible and no
   longer need to iterate across the LC_SEGMENT commands to emit
   LC_DATA_IN_CODE.

2. There's no need to store a pointer to the data in code entries during
   the parse step; we can just look it up as part of the output step.

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D115556