[flang] Accept a separate module procedure interface as a specific procedure

The code snippet

  module m
    interface
      module subroutine specific
      end subroutine
    end interface
    interface generic
       module procedure specific
    end interface
  end module

elicits a bogus semantic error about "specific" not being an acceptable
module procedure for the generic interface; fix.

Differential Revision: https://reviews.llvm.org/D134402