[dsymutil] Don't print swift interface timestamp warning if we have no timestamp

[JDevlieghere]

Summary

If we have no timestamp, don't print the corresponding warning. The binary holder already successfully ignores these cases, but the warning for swift interface files was lacking it.

rdar://86036385

[aprantl]

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

417

We could have a local variable called epochStart or something to make this more readable, but wither way is fine.

llvm/tools/dsymutil/DwarfLinkerForBinary.cpp

if (Obj->getType() == MachO::N_AST) {
         continue;
       }
       if (!Options.NoTimestamp) {
         // The modification can have sub-second precision so we need to cast
         // away the extra precision that's not present in the debug map.
         auto ModificationTime =
             std::chrono::time_point_cast<std::chrono::seconds>(
                 Stat.getLastModificationTime());
-        if (ModificationTime != Obj->getTimestamp()) {
+        if (Obj->getTimestamp() != sys::TimePoint<>() &&
+            ModificationTime != Obj->getTimestamp()) {
           // Not using the helper here as we can easily stream TimePoint<>.
           WithColor::warning()
               << File << ": timestamp mismatch between swift interface file ("
               << sys::TimePoint<>(ModificationTime) << ") and debug map ("
               << sys::TimePoint<>(Obj->getTimestamp()) << ")\n";
           continue;
         }
       }