[Flang] Fix PowerPC build failure due to the deprecation of ArrayRef(std::nullopt_t) {}. (#147816)

Our local Flang build on PowerPC was broken as
```
llvm/flang/../mlir/include/mlir/IR/ValueRange.h:401:20: error: 'ArrayRef' is deprecated: Use {} or ArrayRef<T>() instead [-Werror,-Wdeprecated-declarations]
  401 |       : ValueRange(ArrayRef<Value>(std::forward<Arg>(arg))) {}
      |                    ^
llvm/flang/lib/Optimizer/CodeGen/CodeGen.cpp:2243:53: note: in instantiation of function template specialization 'mlir::ValueRange::ValueRange<const std::nullopt_t &, void>' requested here
 2243 |                              /*cstInteriorIndices=*/std::nullopt, fieldIndices,
      |                                                     ^
 llvm/include/llvm/ADT/ArrayRef.h:70:18: note: 'ArrayRef' has been explicitly marked deprecated here
   70 |     /*implicit*/ LLVM_DEPRECATED("Use {} or ArrayRef<T>() instead", "{}")
      |                  ^
llvm/include/llvm/Support/Compiler.h:244:50: note: expanded from macro 'LLVM_DEPRECATED'
  244 | #define LLVM_DEPRECATED(MSG, FIX) __attribute__((deprecated(MSG, FIX)))
      |                                                  ^
1 error generated.
```

This patch is to fix it.

Author: DanielCChen

flang/lib/Optimizer/CodeGen/CodeGen.cpp

@@ -2239,17 +2239,18 @@ struct XReboxOpConversion : public EmboxCommonConversion<fir::cg::XReboxOp> {
         getSubcomponentIndices(rebox, rebox.getBox(), operands, fieldIndices);
       if (!rebox.getSubstr().empty())
         substringOffset = operands[rebox.getSubstrOperandIndex()];
-      base = genBoxOffsetGep(rewriter, loc, base, llvmBaseObjectType, zero,
-                             /*cstInteriorIndices=*/std::nullopt, fieldIndices,
-                             substringOffset);
+      base =
+          genBoxOffsetGep(rewriter, loc, base, llvmBaseObjectType, zero,
+                          /*cstInteriorIndices=*/llvm::ArrayRef<mlir::Value>(),
+                          fieldIndices, substringOffset);
     }
 
     if (rebox.getSlice().empty())
       // The array section is of the form array[%component][substring], keep
       // the input array extents and strides.
       return finalizeRebox(rebox, adaptor, destBoxTy, dest, base,
-                           /*lbounds*/ std::nullopt, inputExtents, inputStrides,
-                           rewriter);
+                           /*lbounds*/ llvm::ArrayRef<mlir::Value>(),
+                           inputExtents, inputStrides, rewriter);
 
     // The slice is of the form array(i:j:k)[%component]. Compute new extents
     // and strides.
@@ -2297,8 +2298,8 @@ struct XReboxOpConversion : public EmboxCommonConversion<fir::cg::XReboxOp> {
       }
     }
     return finalizeRebox(rebox, adaptor, destBoxTy, dest, base,
-                         /*lbounds*/ std::nullopt, slicedExtents, slicedStrides,
-                         rewriter);
+                         /*lbounds*/ llvm::ArrayRef<mlir::Value>(),
+                         slicedExtents, slicedStrides, rewriter);
   }
 
   /// Apply a new shape to the data described by a box given the base address,
@@ -3396,7 +3397,8 @@ static void genBrOp(A caseOp, mlir::Block *dest, std::optional<B> destOps,
   if (destOps)
     rewriter.replaceOpWithNewOp<mlir::LLVM::BrOp>(caseOp, *destOps, dest);
   else
-    rewriter.replaceOpWithNewOp<mlir::LLVM::BrOp>(caseOp, std::nullopt, dest);
+    rewriter.replaceOpWithNewOp<mlir::LLVM::BrOp>(
+        caseOp, llvm::ArrayRef<mlir::Value>(), dest);
 }
 
 static void genCaseLadderStep(mlir::Location loc, mlir::Value cmp,

flang/lib/Optimizer/CodeGen/PreCGRewrite.cpp

@@ -107,9 +107,11 @@ class EmboxConversion : public mlir::OpRewritePattern<fir::EmboxOp> {
       shapeOpers.push_back(extVal);
     }
     auto xbox = rewriter.create<fir::cg::XEmboxOp>(
-        loc, embox.getType(), embox.getMemref(), shapeOpers, std::nullopt,
-        std::nullopt, std::nullopt, std::nullopt, embox.getTypeparams(),
-        embox.getSourceBox(), embox.getAllocatorIdxAttr());
+        loc, embox.getType(), embox.getMemref(), shapeOpers,
+        llvm::ArrayRef<mlir::Value>(), llvm::ArrayRef<mlir::Value>(),
+        llvm::ArrayRef<mlir::Value>(), llvm::ArrayRef<mlir::Value>(),
+        embox.getTypeparams(), embox.getSourceBox(),
+        embox.getAllocatorIdxAttr());
     LLVM_DEBUG(llvm::dbgs() << "rewriting " << embox << " to " << xbox << '\n');
     rewriter.replaceOp(embox, xbox.getOperation()->getResults());
     return mlir::success();