[flang] Error message situation should be a warning

f18 emits an error message when the same name is used in a scope
for both a procedure and a generic interface, and the procedure is
not a specific procedure of the generic interface.  It may be
questionable usage, and not portable, but it does not appear to
be non-conforming by a strict reading of the standard, and many
popular Fortran compilers accept it.

Differential Revision: https://reviews.llvm.org/D135205