[lld/mac] Move handling of special undefineds later

treatUndefinedSymbol() was previously called before gatherInputSections()
and markLive() for these special symbols, but after them for normal
undefineds.

For PR50760, treatUndefinedSymbol() will have to potentially create
sections, so it's good to move treatUndefinedSymbol() for special
undefineds later, so that it can assume that gatherInputSections()
and markLive() has already been called always.

No intended behavior change, but part of PR50760 (and covered in
tests in the patch for the full feature).

Differential Revision: https://reviews.llvm.org/D106552