[flang] Downgrade error to warning for consistency (#157191)
When a procedure name is declared EXTERNAL and then followed up with an
explicit INTERFACE, the compiler emits an error when the procedure is a
dummy procedure but only a warning otherwise. Since the EXTERNAL
statement is harmless in both cases, accept the case of a dummy
procedure as well for consistency.
Fixes https://github.com/llvm/llvm-project/issues/157162.