[Async CC] Fixed ptrauth for dynamic ptrs in partial applies.
A partial apply of an async non-direct function entails storing a
pointer to the function's AsyncFunctionPointer into the thick context
using a specific (IGM.getOptions().PointerAuth.PartialApplyCapture)
ptrauth schema. Previously, the incorrect schema (derived from the
function type) was used to auth the ptr-to-AsyncFunctionPointer and then
to again sign the ptr-to-function. Here that error is corrected. Now,
the pointer-to-AsyncFunctionPointer is auth'd using that specific schema
and then the extracted function pointer is again signed again using it.
