[flang] optionally lower scalar and explicit shape with fir.declare

Lower scalar and explicit shape arrays to fir.declare under the -hlfir option.
Update the SymMap so that it can hold fir::FortranVariableInterface.

The plan is to go towards a SymMap that only contains fir::FortranVariableInterface
once current expression lowering can be replaced. This should make the SymMap lighter
than it is today (SymBox/ExtendedValue are above 256 bytes).

Assumed shape, allocatable and pointer are left TODOs for now. Anything with a
specification expression that is not a constant expression will only be able to
be lowered when the HLFIR expression lowering skeleton is added.

Differential Revision: https://reviews.llvm.org/D136252