[flang] Name resolution for Cray pointers

Resolve the pointer and pointee names in a `BasedPointerStmt` and
enforce some of the constraints on them. There are still some
constraints to be implemented, mainly about what kind of attributes
the pointers and pointees can have.

The rules for these are a little vague. I mostly followed
- Cray Fortran Reference Manual section 9.3.2
- https://gcc.gnu.org/onlinedocs/gfortran/Cray-pointers.html
- VSI Fortran for OpenVMS Language Reference Manual section B.11

Note that the first two use the term "Cray pointer" but the last does
not. That is confusing because you have to know from context whether
it is referring to Cray pointers or Fortran pointers, so I used
"Cray pointer" and "Cray pointee" in error messages to refer to the
two names in the pointer statement.

Original-commit: flang-compiler/f18@cabb112be2922ee2be9e29cfedcc30053c9a2864
Reviewed-on: https://github.com/flang-compiler/f18/pull/697