Updates codegen for global destructors and raising exceptions to ensure
that the function pointers being passed are signed using the correct
schema.
Notably this requires that CodeGenFunction::createAtExitStub to return
an opaque Constant* rather than a Function* as the value being emitted
is no longer necessarily a raw function pointer depending on the
configured ABI.
Co-Authored-By: Akira Hatanaka <ahatanaka@apple.com>
Co-Authored-By: John McCall <rjmccall@apple.com>
