Improve checker for ReferenceModifier

[fm-117]

IBM compiler allows decimal values in reference modifiers and emits an error when using negative values.

In our current implementation, no diagnostic is created for both of these cases. To improve our error reporting, we should:

• emit an error on negative index/length (same for zero index or length)
• emit a warning on decimal index/length

Code to check:

           MOVE item(0:5) TO part
           MOVE item(1:0) TO part
           MOVE item(0:0) TO part
           MOVE item(-1:5) TO part
           MOVE item(1:-5) TO part
           MOVE item(-1:-5) TO part
           MOVE item(1.32:5) TO part
           MOVE item(1:5.84) TO part
           MOVE item(1.32:5.84) TO part
           MOVE item(0.0:5) TO part
           MOVE item(1:0.0) TO part
           MOVE item(0.0:0.0) TO part
           MOVE item(-1.32:5) TO part
           MOVE item(1:-5.84) TO part
           MOVE item(-1.32:-5.84) TO part

[fm-117]

@delevoye Could you check diagnostics reported by IBM compiler when using variables ? Does it also check initial values ?