[lld-macho] Mark weak symbols in symbol table

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D86908

lld/MachO/SyntheticSections.cpp

@@ -473,10 +473,11 @@ void SymtabSection::writeTo(uint8_t *buf) const {
   for (const SymtabEntry &entry : symbols) {
     nList->n_strx = entry.strx;
     // TODO support other symbol types
-    // TODO populate n_desc
+    // TODO populate n_desc with more flags
     if (auto *defined = dyn_cast<Defined>(entry.sym)) {
       nList->n_type = MachO::N_EXT | MachO::N_SECT;
       nList->n_sect = defined->isec->parent->index;
+      nList->n_desc |= defined->isWeakDef() ? MachO::N_WEAK_DEF : 0;
       // For the N_SECT symbol type, n_value is the address of the symbol
       nList->n_value = defined->value + defined->isec->getVA();
     }

lld/test/MachO/symtab.s

@@ -20,7 +20,8 @@
 # CHECK-NEXT:     Type: Section (0xE)
 # CHECK-NEXT:     Section: __text (0x1)
 # CHECK-NEXT:     RefType:
-# CHECK-NEXT:     Flags [ (0x0)
+# CHECK-NEXT:     Flags [ (0x80)
+# CHECK-NEXT:       WeakDef (0x80)
 # CHECK-NEXT:     ]
 # CHECK-NEXT:     Value:
 # CHECK-NEXT:   }
@@ -42,6 +43,7 @@ foo:
   .asciz "Hello world!\n"
 
 .text
+.weak_definition bar
 .global bar
 .global _main