Account for back-deployed concurrency library

The back-deployed concurrency library, which is used on OS versions
prior to the introduction of concurrency, has a Swift 5.6-era
concurrency library that supports vouchers.

Author: DougGregor

stdlib/toolchain/Compatibility56/Concurrency/Actor.cpp

@@ -152,9 +152,14 @@ static bool vouchersDisabled;
 
 static void _initializeVouchersDisabled(void *ctxt) {
   if (__builtin_available(macOS 12.1, iOS 15.2, tvOS 15.2, watchOS 8.3, *)) {
+    // Concurrency library in the OS in new enough that it has voucher support.
     vouchersDisabled = false;
-  } else {
+  } else if (__builtin_available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)) {
+    // Concurrency library in the OS falls in the range that has no voucher support.
     vouchersDisabled = true;
+  } else {
+    // Concurrency library is back-deployed on this OS, and has voucher support.
+    vouchersDisabled = false;
   }
 }