[flang] simplify derived type info table format

- Replace class(*) member by a c_ptr member to avoid having to handle
  polymorphic components in the type info table generation. Polymorphic
  entity handling will require these very tables to be lowered properly.
  Note: keep the init as NullPointer/Designators. This is technically
  invalid Fortran, the init should have c_ptr type. But wrapping this
  in a C_LOC intrinsic call would make runtime generation and lowering
  more complex with no real benefits.

- ComponentIterator is crashing when used on the generated derived
  types in GetScope. This patch makes GetScope more robust, but it
  is not entirely clear to me why this is only happening with the
  generated derived types.

- The type of generated character globals was incorrect because
  Scope::FindType was matching character types with different
  length. Add a CharacterTypeSpec == operator to fix this.

Differential Revision: https://reviews.llvm.org/D102768