[lld-macho] Handle alignment correctly when merging InputSections

Summary:
Previously, we weren't updating isecAddr when aligning InputSections,
resulting in truncated sections under the right conditions.

Reviewers: #lld-macho, compnerd

Reviewed By: #lld-macho, compnerd

Subscribers: smeenai, compnerd, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81298