[flang-commits] [flang] [flang] Downgrade error to warning for consistency (PR #157191)

[Kelvin Li via flang-commits]

kkwli wrote:

If `f` is not a dummy procedure, the warning is issue.
```fortran
      interface
        subroutine g()
          external f
          interface
            subroutine f()
            end
          end interface
        end
      end interface
      end
```
Output:
```
./dd2.f90:5:24: warning: EXTERNAL attribute was already specified on 'f' [-Wredundant-attribute]
              subroutine f()
                         ^
```

https://github.com/llvm/llvm-project/pull/157191