[flang] Extension: forward refs to dummy args under IMPLICIT NONE

Most Fortran compilers accept the following benign extension,
and it appears in some applications:

  SUBROUTINE FOO(A,N)
    IMPLICIT NONE
    REAL A(N) ! N is used before being typed
    INTEGER N
  END

Allow it in f18 only for default integer scalar dummy arguments.

Differential Revesion: https://reviews.llvm.org/D96982