[flang] Introduce FortranVariableOpInterface for ops creating variable

HLFIR will rely on certain operations to create SSA memory values
that correspond to a Fortran variable. They will hold bounds and type
parameters information as well as metadata (like Fortran attributes).

This patch adds an interface that for such operations so that Fortran
variable can be stored, manipulated, and queried regardless of what
created them. This is so far intended for fir.declare, hlfir.designate
and hlfir.associate operations.
It is added to FIR and not HLFIR because fir.declare needs it and it
does not itself needs any HLFIR concepts.

Unit tests for the interface methods will be added alongside
fir.declare in the next patch.

Differential Revision: https://reviews.llvm.org/D136151