[flang] Foil attempts to use C_PTR/C_FUNPTR as structure constructors

The internal details of the C_PTR and C_FUNPTR types must be made private
so that user code can't try to access their components or attempt to use
their structure constructors.

Fixes llvm-test-suite/Fortran/fortran/c_ptr_tests_13.f90.

Differential Revision: https://reviews.llvm.org/D157343