[flang] Correct procedure pointer (or dummy) compatibility check

Fix a subtle bug in procedure compatibility checking with base
derived types vs. their extensions to ensure that a procedure
expecting an extended type cannot be associated with a pointer
(or dummy procedure) to a procedure expecting a base type.

  subroutine s1(base); ... subroutine s2(extended)
  procedure(s1), pointer :: p
  p => s2 ! <- must be caught as an error

Differential Revision: https://reviews.llvm.org/D142753