Lower elemental subroutine calls (outside of user assignment case)

[jeanPerier]

Use the array lowering framework to lower elemental subroutine.
This required:

• Splitting the part of genIterSpace that creates the
  implicit loops into a new genImplicitLoops function. genIterspace
  deals with the destination and other explicit contexts set-ups that
  do not matter in the subroutine case (there is no destination).
  Other than having no destination, a big difference is that there is
  no threaded innerArg. genImplicitLoops handles this variation.

• Fixing a bug in the iteration shape determination. The code was
  assuming there would always be at least one array_load, which is
  not true if there are only RefTransparent operands. This is always
  true in the subroutine case, but was actually also an issue for things
  like print *, elem_func(array) with the previous code (compile crash).
  Add a new ArrayOperands structure to keep track of what matters to
  later deduce the shape of the array expression instead of keeping
  track of array_loads for that purpose.

• Elemental subroutine with intent(out)/intent(inout) arguments must be
  applied "in array element order" (15.8.3). It turns out this is also
  the case for impure elemental functions (10.1.4 p5). Instead of always
  creating loops as "unordered=true" add an unordered field to the ArrayExprLowering
  class. It is set to true by default, but will be set to false if any
  impure elemental function is lowered, or in case of
  intent(out)/intent(inout) arguments in elemental subroutines.

• Last, instead of using createFirExpr to lowering subroutine calls, use
  a new createSubroutineCall that deals with the array/scalar
  dispatching.

A TODO is added for the user defined elemental assignment, because
overlap analysis between RHS/LHS must be done, and this require somehow
plugging this special and very restricted case of elemental calls into
the array_load/array_update/array_merge framework or some ad-hoc
lowering.

Note: some dead code in the code that became genImplicitLoops is removed here (FORALL refactoring leftover ?). The loop bounds are now evaluated after the ccPrelude mask due to the genImplicitLoops code move, hence the forall lit test change.

[psteinfeld]

I don't really understand the code, so someone who does should approve.

I've verified that everything builds and tests correctly.

[clementval]

LGTM