[flang][hlfir] Array constructor lowering [part 2/4]

This patch adds the lowering strategy that lowers an array constructor
to an hlfir.elemental (without creating any temporary yet in lowering).
This will allow more high level array expression optimization to elide
the array constructor temporary when possible, but this is only doable
for a restricted although common form of array constructors:
"[(pure_scalar_expr(i),i=lower,upper,stride)]".

Differential Revision: https://reviews.llvm.org/D144111