[flang] Accept assumed shape arrays as SHAPE in C_F_POINTER

C_F_POINTER was added in https://reviews.llvm.org/D132303, but the code
assumed that SHAPE would always be an explicit shape with compile time
constant rank. It can actually be an assumed shape, or an explicit shape
with non compile time constant rank. Get the rank from FPTR pointer
instead.

Differential Revision: https://reviews.llvm.org/D133347