[PtrAuth] Corrected AsyncContextParent schema.

Previously, the schema used a key intended for pointers to code.  An
AsyncContextParent is actually a pointer to data, so that key was
incorrect.  Here, that error is corrected and the schema now uses the
key for pointers to data.

Author: nate-chandler

lib/IRGen/IRGen.cpp

@@ -690,7 +690,7 @@ static void setPointerAuthOptions(PointerAuthOptions &opts,
       SpecialPointerAuthDiscriminators::ResilientClassStubInitCallback);
 
   opts.AsyncContextParent =
-      PointerAuthSchema(codeKey, /*address*/ true, Discrimination::Constant,
+      PointerAuthSchema(dataKey, /*address*/ true, Discrimination::Constant,
                         SpecialPointerAuthDiscriminators::AsyncContextParent);
 
   opts.AsyncContextResume =