[lld-macho][nfc] Remove file statics from ICF.cpp

This gets us closer to the [LLD-as-a-library goal][1].

[1]: https://lists.llvm.org/pipermail/llvm-dev/2021-June/151184.html

Reviewed By: #lld-macho, thakis

Differential Revision: https://reviews.llvm.org/D121050