[lld] [lld][MachO] Fix symbol insertion in `transplantSymbolsAtOffset` (PR #120737)

[Carlo Cabrera via llvm-commits]

carlocab wrote:

Tried this:
```diff
diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
index a61e60a944fb..4b6e462f6bdd 100644
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -68,7 +68,7 @@ static void transplantSymbolsAtOffset(InputSection *fromIsec,
                                       uint64_t fromOff, uint64_t toOff) {
   // Ensure the symbols will still be in address order after our insertions.
   auto symSucceedsOff = [](uint64_t off, const Symbol *s) {
-    return cast<Defined>(s)->value > off;
+    return cast<Defined>(s)->value < off;
   };
   assert(std::is_partitioned(toIsec->symbols.begin(), toIsec->symbols.end(),
                              [symSucceedsOff, toOff](const Symbol *s) {
@@ -96,6 +96,11 @@ static void transplantSymbolsAtOffset(InputSection *fromIsec,
     }
     return true;
   });
+  assert(llvm::is_sorted(toIsec->symbols,
+                         [](const Defined *s, const Defined *t) {
+                           return s->value < t->value;
+                         }) &&
+         "Symbols should still be sorted at exit.");
 }
 
 Defined *SymbolTable::addDefined(StringRef name, InputFile *file,
diff --git a/lld/test/MachO/alias-symbols.s b/lld/test/MachO/alias-symbols.s
index 44a388c35722..231ada5aa4c8 100644
--- a/lld/test/MachO/alias-symbols.s
+++ b/lld/test/MachO/alias-symbols.s
@@ -124,7 +124,9 @@ _strong:
 _weak_1:
   .space 1
 _weak_2:
-  .space 1
+  .long 1
+_weak_3:
+  .long 1
 _dead:
   .space 1
 _pext:
```
Still no failure from `lld/test/MachO/alias-symbols.s`. Given that the `is_partitioned` assertion isn't even firing here, I wonder if trying to modify `alias-symbols.s` is going to be a lot more complicated than working with the other tests that are failing.

https://github.com/llvm/llvm-project/pull/120737