[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