[flang][hlfir] Lower left-hand side vector subscripts to HLFIR

This patch lowers assignments to vector subscripted designators into the
newly added hlfir.elemental_addr and hlfir.region_assign.

Note that the codegen of these operation to FIR is still TODO and will
still emit a TODO message when trying to compile programs end to end.

Differential Revision: https://reviews.llvm.org/D149962