[lld/mac] Move output segment rename logic into OutputSegment

Fixes the output segment name if both -rename_section and
-rename_segment are used and the post-section-rename segment
name is the same as the pre-segment-rename segment name to
match ld64's behavior.

The motivation is that segment$start$ can create section-less segments,
and this makes a corner case in the interaction between segment$start and
-rename_segment in the upcoming segment$start patch.

Differential Revision: https://reviews.llvm.org/D106766