[flang] Fix wording of warning message

[klausler]

Summary

"division on intrinsic call" should read "division by zero on intrinsic call".

[clementval]

LGTM

flang/lib/Evaluate/common.cpp

void RealFlagWarnings(
     FoldingContext &context, const RealFlags &flags, const char *operation) {
   if (flags.test(RealFlag::Overflow)) {
     context.messages().Say("overflow on %s"_warn_en_US, operation);
   }
   if (flags.test(RealFlag::DivideByZero)) {
     if (std::strcmp(operation, "division") == 0) {
       context.messages().Say("division by zero"_warn_en_US);
     } else {
-      context.messages().Say("division on %s"_warn_en_US, operation);
+      context.messages().Say("division by zero on %s"_warn_en_US, operation);
     }
   }
   if (flags.test(RealFlag::InvalidArgument)) {
     context.messages().Say("invalid argument on %s"_warn_en_US, operation);
   }
   if (flags.test(RealFlag::Underflow)) {
     context.messages().Say("underflow on %s"_warn_en_US, operation);
   }