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

[Kelvin Li via flang-commits]

kkwli wrote:

Sorry, I miss this case. Should it be covered in this fix?
```fortran
      interface
        subroutine g(f)
          external f
          interface
            subroutine f()
            end
          end interface
        end
      end interface
      end
```
Output:
```
error: Semantic errors in dd2.f90
./dd2.f90:5:24: error: 'f' is already declared in this scoping unit
              subroutine f()
                         ^
./dd2.f90:2:22: Previous declaration of 'f'
          subroutine g(f)
                       ^
```

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