* Remove RegisterPreservingCC. It was unused.
* Remove DefaultCC from the runtime. The distinction between C_CC and DefaultCC
was unused and inconsistently applied. Separate C_CC and DefaultCC are
still present in the compiler.
* Remove function pointer indirection from runtime functions except those
that are used by Instruments. The remaining Instruments interface is
expected to change later due to function pointer liability.
* Remove swift_rt_ wrappers. Function pointers are an ABI liability that we
don't want, and there are better ways to get nonlazy binding if we need it.
The fully custom wrappers were only needed for RegisterPreservingCC and
for optimizing the Instruments function pointers.
This is different from swift_deallocObject in that it applies to objects
at +1 while swift_deallocObject actually only applies to objects whose
state is deiniting (swift_release was called).
When a boxed type is destroyed, ensure that the object is marked as
deiniting first. This was caught by an assertion in the runtime.
Addresses SR-6268!
