[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.
llvm-project
13ead000 - [Flang] Fix PowerPC build failure due to the deprecation of ArrayRef(std::nullopt_t) {}. (#147816)
