CPP12 — semantics of valuetype _downcast operation unclear

The definition of the _downcast operation generated for valuetypes is
unclear about whether it increments the reference count of the valuetype
before returning the downcasted pointer. I assume by the fact that it
is called _downcast rather than _narrow that it should not increment the
reference count.

Proposal:

Add the following text to the end of the last paragraph in 1.17.3:

"The _downcast function does not increment the reference count of the
valuetype."

Add the proposed text