[flang] Pointer assignment semantic checks

Call `CheckPointerAssignment()` when analyzing a pointer assignment
statement. NOTE: the cases with bounds-spec and bounds-remapping are
still to be done.

Perform checks on pointer symbols in `check-declarations.cc`.

Check for pointer to generic intrinsic in `semantics/expression.cc`.

Add the other required pointer assignment checks to `pointer-assignment.cc`.

Original-commit: flang-compiler/f18@3dc5fd6d9e58d1ef0efd1deefcbaa52499ad93f9
Reviewed-on: https://github.com/flang-compiler/f18/pull/928