[flang][hlfir] Add codegen for vector subscripted LHS

This patch adds support for vector subscripted assignment left-hand
side. It does not yet add support for the cases where the LHS must be
saved because its evaluation could be impacted by the assignment.

The implementation adds an hlfir::ElementalOpInterface to share the
elemental inlining utility and some other tools between
hlfir::ElementalOp and hlfir::ElelemntalAddrOp.

It adds generateYieldedLHS() to allow retrieving the LHS value
in lowering, whether or not it is vector subscripted. If it is vector
subscripted, this utility creates a loop nest iterating over the
elements and returns the address of an element.

Differential Revision: https://reviews.llvm.org/D153759