Pointer auth protection of the block descriptor pointer is only
supported in some constrained environments so we do actually need it to
be configurable.
We had made it non configurable in the first PR to protect block
metadata because we believed that was an option but subsequently
realised it does need to remain configurable.
This PR revives the flags that permit this.
Introduces the use of pointer authentication to protect the invocation,
copy and dispose, reference, and descriptor pointers in Objective-C
block objects.
Resolves#141176
This PR introduces the use of pointer authentication to objective-c[++].
This includes:
* __ptrauth qualifier support for ivars
* protection of isa and super fields
* protection of SEL typed ivars
* protection of class_ro_t data
* protection of methodlist pointers and content
