[flang][hlfir] lower hlfir.shape_of

If possible the shape is gotten from the bufferization of the expr
argument.

The simple cases should already have been resolved during lowering. This
is mostly intended for cases where shape information is added in between
lowering and the end of bufferization (for example transformational
intrinsics with assumed shape arguments).

Depends on: D146832

Differential Revision: https://reviews.llvm.org/D146833