[concurrency] SILGen: allow the Builtin.createAsyncTaskFuture to have a non-generic closure argument.

It fixes a crash in SILGen if Builtin.createAsyncTaskFuture is used in a non-generic context.
I found this by experiment - we don't use it currently in the stdlib. But it doesn't harm to fix this.

Author: eeckstein

lib/SILGen/SILGenBuiltin.cpp

@@ -1439,7 +1439,7 @@ static ManagedValue emitBuiltinCreateAsyncTaskFuture(
   // Form the metatype of the result type.
   CanType futureResultType =
       Type(
-        MetatypeType::get(GenericTypeParamType::get(0, 0, SGF.getASTContext())))
+        MetatypeType::get(GenericTypeParamType::get(0, 0, SGF.getASTContext()), MetatypeRepresentation::Thick))
           .subst(subs)->getCanonicalType();
   CanType anyTypeType = ExistentialMetatypeType::get(
       ProtocolCompositionType::get(ctx, { }, false))->getCanonicalType();