[flang] Move pointer assignment checking to its own file

Create `pointer-assignment.{h,cc}` for pointer assignment checking.
It doesn't share with assignment checking so it should be its own file.
Move the code into semantics namespace.

Original-commit: flang-compiler/f18@1658aba81f24fe3913298e76ae8b9c938bd4d248
Reviewed-on: https://github.com/flang-compiler/f18/pull/928
Tree-same-pre-rewrite: false