[flang] Add hlfir.declare operation

This operation will be used to declare named variables in HLFIR.
See the added description in HLFIROpBase.td for more info about it.

The motivation behind this operation is described in https://reviews.llvm.org/D137634.

The FortranVariableInterface verifier is changed a bit. It used to
operate using the result type to verify the provided shape and length
parameters. This is a bit incorrect because what matters to verify the
information is the input address (This worked OK with fir.declare where
the input memref type is the same as the output result). Also, not all
operation defining variables will have an input memref with the same
meaning (hlfir.designate and hlfir.associate for instance).
Hence, this verifier is now optional and must be provided a memref to
operate.

Differential Revision: https://reviews.llvm.org/D137781