[flang] Separate module procedure variant

Accept "module procedure" (as well as module function/subroutine)
in a separate module procedure definition, such as "bb1" in:

module mm
  interface
    module subroutine mm1
    end subroutine
  end interface
end module

submodule(mm) bb
  interface
    module subroutine bb1
    end subroutine
  end interface
contains
  module procedure mm1
    call bb1
  end procedure
  module procedure bb1
    print*, 'bb1'
  end procedure
end submodule

  use mm
  call mm1
end