[flang] Add hlfir.designate definition

Add hlfir.designate operation definition. This is the HLFIR building
blocks to represent Fortran designator. An hlfir.designator is a
Fortran "part-ref" and an optional susbstring or part ref.
See the operation added description for more detail, and
https://github.com/llvm/llvm-project/blob/main/flang/docs/HighLevelFIR.md
for the rational of this operation.

Differential Revision: https://reviews.llvm.org/D138121