[flang] Don't warn about a NaN argument to ISNAN()

[klausler]

Summary

Don't emit a warning when a compile-time constant argument to
ISNAN() or IEEE_IS_NAN() is a NaN.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/fold-logical.cpp

           return Expr<T>{*result};
         }
       }
     }
   } else if (name == "isnan" || name == "__builtin_ieee_is_nan") {
-    using DefaultReal = Type<TypeCategory::Real, 4>;
     // Only replace the type of the function if we can do the fold
     if (args[0] && args[0]->UnwrapExpr() &&
         IsActuallyConstant(*args[0]->UnwrapExpr())) {
+      auto restorer{context.messages().DiscardMessages()};
+      using DefaultReal = Type<TypeCategory::Real, 4>;
       return FoldElementalIntrinsic<T, DefaultReal>(context, std::move(funcRef),
           ScalarFunc<T, DefaultReal>([](const Scalar<DefaultReal> &x) {
             return Scalar<T>{x.IsNotANumber()};
           }));
     }

flang/test/Evaluate/errors01.f90

     real, parameter :: bad3 = dim(huge(1.),-.5*huge(1.))
     !CHECK: warning: DIM intrinsic folding overflow
     integer, parameter :: bad4 = dim(huge(1),-1)
     !CHECK: warning: overflow on REAL(8) to REAL(4) conversion
     x = 1.D40
+    !CHECK-NOT: warning: invalid argument
+    if (.not. isnan(real(z'ffffffffffffffff',8))) stop
   end subroutine
 end module