[flang] Foil attempts to require interoperable pointers be CONTIGUOUS

BIND(C) interoperable pointer descriptors may not be required to be
CONTIGUOUS in procedure interfaces.

(Also fixed erroneous true result from IsDescriptor() predicate for
assumed-size arrays that was exposed by testing.)

Fixes llvm-test-suite/Fortran/gfortran/regression/bind_c_contiguous.f90.

Differential Revision: https://reviews.llvm.org/D157342