[flang][hlfir] Added allocatable/pointer support in hlfir::convertToValue.

The code is used, for example, when passing arguments to IO or intrinsic
calls as value. The allocatable/pointer boxes must be dereferenced,
and trivial values have to be loaded. Character and derived values
have to stay boxed.
I am not sure what to do for the array cases, and I have not seen
any test triggering it, so I leave it as a TODO.

Reviewed By: tblah, clementval

Differential Revision: https://reviews.llvm.org/D151925