[lld-macho] Prevent assertions for aliases to weak_def_can_be_hidden symbols

In https://reviews.llvm.org/D137982 we found that on Mach-O private
aliases could trigger an assert in lld when the aliasee was a
weak_def_can_be_hidden symbol.

This appears to be incorrect, and should be allowed in Mach-O.
Disallowing this behavior is also inconsistent with how ld64 handles
a private alias to weak_def_can_be_hidden symbols.

This patch removes the assert and tests that LLD handles such aliases
gracefully.

Reviewed By: #lld-macho, int3

Differential Revision: https://reviews.llvm.org/D141082

lld/MachO/InputFiles.cpp

@@ -698,8 +698,6 @@ static macho::Symbol *createDefined(const NList &sym, StringRef name,
         sym.n_desc & REFERENCED_DYNAMICALLY, sym.n_desc & N_NO_DEAD_STRIP,
         isWeakDefCanBeHidden);
   }
-  assert(!isWeakDefCanBeHidden &&
-         "weak_def_can_be_hidden on already-hidden symbol?");
   bool includeInSymtab = !isPrivateLabel(name) && !isEhFrameSection(isec);
   return make<Defined>(
       name, isec->getFile(), isec, value, size, sym.n_desc & N_WEAK_DEF,