[IRGen] Mark protocol requirement descriptor for distributed thunk as async

Fixes a bug where descriptor for protocol requirement associated with
distributed thunk wasn't marked as `async` that results in ptrauth failures
because discriminator would be misaligned between requirement and witness.

Resolves: rdar://96520492

Author: xedin

lib/IRGen/GenMeta.cpp

@@ -913,6 +913,10 @@ namespace {
         if (entry.getFunction().isAutoDiffDerivativeFunction())
           declRef = declRef.asAutoDiffDerivativeFunction(
               entry.getFunction().getAutoDiffDerivativeFunctionIdentifier());
+        if (entry.getFunction().isDistributedThunk()) {
+          flags = flags.withIsAsync(true);
+          declRef = declRef.asDistributed();
+        }
         addDiscriminator(flags, schema, declRef);
       }