[flang] c_funloc - handle pocedure pointers in convertToBox (#76070)

C_FUNLOC was not handling procedure pointer argument correctly, the
issue lied in `hlfir::convertToBox` that did not handle procedure
pointers.

I modified the interface of `hlfir::convertToXXX` to take values on the
way because hlfir::Entity are fundamentally an mlir::Value with type
guarantees, so they should be dealt with by value as mlir::Value are
(they are very small).