[flang][openacc] Add error for unsupported min/max reduction on complex type

[clementval]

Summary

reduction operator min and max are not supported with complex type.
Add a proper error so it fails correctly.

[razvanlupusoru]

Looks good. Though it makes me wonder if we should be stricter than just min/max. Other reduction operators including logical don't really apply.

[clementval]

In D155016#4491341, @razvanlupusoru wrote:

Looks good. Though it makes me wonder if we should be stricter than just min/max. Other reduction operators including logical don't really apply.

I'll add a semantic check to provide better error message.

flang/lib/Lower/OpenACC.cpp

if (op != mlir::acc::ReductionOperator::AccAdd &&
     TODO(loc, "reduction operator");
 
   if (ty.isIntOrIndex())
     return builder.create<mlir::arith::ConstantOp>(
         loc, ty,
         builder.getIntegerAttr(ty, getReductionInitValue<llvm::APInt>(op, ty)));
   if (op == mlir::acc::ReductionOperator::AccMin ||
       op == mlir::acc::ReductionOperator::AccMax) {
+    if (mlir::isa<fir::ComplexType>(ty))
+      llvm::report_fatal_error(
+          "min/max reduction not supported for complex type");
     if (auto floatTy = mlir::dyn_cast_or_null<mlir::FloatType>(ty))
       return builder.create<mlir::arith::ConstantOp>(
           loc, ty,
           builder.getFloatAttr(ty,
                                getReductionInitValue<llvm::APFloat>(op, ty)));
   } else {
     if (auto floatTy = mlir::dyn_cast_or_null<mlir::FloatType>(ty))
       return builder.create<mlir::arith::ConstantOp>(