[flang][hlfir] add hlfir.get_extent

This operation fetches an extent value from a fir.shape. The operation
could just as easily live in the fir namespace, but is only needed for
hlfir lowering so I put it here.

This operation is required to allow one to defer getting the extents of a shape
generated by hlfir.get_shape until after that shape has been resolved
(after bufferization of the hlfir.expr).

This operation will be lowered to FIR as an arith.constant created using
the definition of the fir.shape argument.

Depends on: D146830

Differential Revision: https://reviews.llvm.org/D148220