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

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.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D128922

Co-authored-by: Peter Steinfeld <psteinfeld@nvidia.com>

flang/lib/Lower/IntrinsicCall.cpp

@@ -4063,7 +4063,7 @@ static mlir::Value createExtremumCompare(mlir::Location loc,
   } 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.