[flang] Downgrade error message to warning (#108115)
It is a non-mandatory error to reference an external procedure via an
implicit interface declaration (EXTERNAL or PROCEDURE()) when the
external procedure has an interface that requires the presence of an
explicit interface to be called.
Until now, the compiler has issued a fatal error message from semantics
for this situation. But (1) there are situations, such as passing such
an EXTERNAL as an actual argument, or as the target of a procedure
pointer assignment, where little or no harm is done, (2) other compilers
don't/can't detect this error, even when the procedure's definition is
in the same source file, and (3) it shows up in some real applications.
So downgrade this error to a stern warning. Perhaps in the future the
compiler could resume emission of a hard error in the cases where the
EXTERNAL procedure is actually known to be called via its implicit
interface.
