[lld-macho] `-exported_symbols` should hide symbols before LTO runs

We were previously doing it after LTO, which did have the desired effect
of having the un-exported symbols marked as private extern in the final
output binary, but doing it before LTO creates more optimization
opportunities.

One observable difference is that LTO can now elide un-exported symbols
entirely, so they may not even be present as private externs in the
output.

This is also what ld64 implements.

Reviewed By: #lld-macho, thevinster

Differential Revision: https://reviews.llvm.org/D130429