[flang][hlfir] Support type bound procedure in type descriptors

In hlfir, procedure designators are propagated as fir.box_proc.
Derived type descriptors are compiler generated constant structure
constructors. They contain CFPTR components for the type bound
procedure addresses.
Before being cast to an integer type so that they can be stored
in the CFPTR components, the fir.box_proc addresses must be
obtained with a fir.box_addr.

Differential Revision: https://reviews.llvm.org/D144952