[flang] Fix "not yet implemented" message for CHARACTER MIN/MAX

[clementval]

Summary

The check to see if the arguments for the MIN/MAX intrinsics were of CHARACTER
type was not handling assumed length characters. In this case, the FIR type is
"!fir.ref<!fir.char<1,?>>".

This patch is part of the upstreaming effort from fir-dev branch.

Co-authored-by: Peter Steinfeld <[e-mail]>

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/IntrinsicCall.cpp

if (fir::isa_real(type)) {
     } else {
       // TODO: ieeeMinNum/ieeeMaxNum
       static_assert(behavior == ExtremumBehavior::IeeeMinMaxNum,
                     "ieeeMinNum/ieeeMaxNum behavior not implemented");
     }
   } else if (fir::isa_integer(type)) {
     result =
         builder.create<mlir::arith::CmpIOp>(loc, integerPredicate, left, right);
-  } else if (fir::isa_char(type)) {
+  } else if (fir::isa_char(type) || fir::isa_char(fir::unwrapRefType(type))) {
     // TODO: ! character min and max is tricky because the result
     // length is the length of the longest argument!
     // So we may need a temp.
     TODO(loc, "CHARACTER min and max");
   }
   assert(result && "result must be defined");
   return result;
 }