[flang] Lower component-ref to hlfir.designate

[flang] Lower component-ref to hlfir.designate

Implement the visit of component refs in DesignatorBuilder.
The ArrayRef code has to be updated a bit to cope with the
case where the base is an array and the component is also an
array.

Improve the result type of array sections designators (only return
a fir.box if the array section is not contiguous/has dynamic extent).
This required exposing IsContiguous entry point for different
front-end designator nodes (the implementation already existed,
but was internal to check-expression.cpp).

Differential Revision: https://reviews.llvm.org/D141470