[lld-macho] Handle error cases properly for -exported_symbol(s_list)

This fixes defects in D98223 [lld-macho] implement options -(un)exported_symbol(s_list):
* disallow export of hidden symbols
* verify that whitelisted literal names are defined in the symbol table
* reflect export-status overrides in `nlist` attribute of `N_EXT` or `N_PEXT`

Thanks to @thakis for raising these issues

Differential Revision: https://reviews.llvm.org/D98381