[flang] Fixed IsContiguous check for slices of parameter arrays.

A section of a parameter array may be non-contiguous,
so the current !IsVariable(expr) check is too optimistic
to claim contiguity.

This patch fixes issues with incorrect hlfir.designate op generated
during lowering: the lowering queries IsContiguous to decide whether
to use fir.box<fir.array> or plain fir.ref<fir.array> to represent
the designator result.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D156494