[flang] Add fir.declare operation

Add fir.declare operation whose purpose was described in https://reviews.llvm.org/D134285.
It uses the FortranVariableInterfaceOp for most of its logic (including the verifier).
The rational is that all these aspects/logic will be shared by hlfir.designate and
hlfir.associate.

Its codegen and lowering will be added in later patches.

Differential Revision: https://reviews.llvm.org/D136181