[flang] Roll up fixes for semantic analysis of SPEC CPU codes

- Add cases to IsDescriptor
- Fix NULLIFY in PURE checking
- Fix fotonik3d mutually-referencing derived type bug
- Fix spurious I/O error message on ALLOCATE

Original-commit: flang-compiler/f18@34b64db7da9e90109b1dfec5347b344eae7dd8ad
Reviewed-on: https://github.com/flang-compiler/f18/pull/883