apacheGH-39333: [C++] Don't use "if constexpr" in lambda

It seems that it's not portable. At least it doesn't work as expected
with Visual Studio 2017:

    C:/arrow/cpp/src/arrow/array/array_nested.cc(291): error C2065: 'validity': undeclared identifier (compiling source file C:\arrow-build\src\arrow\CMakeFiles\arrow_shared.dir\Unity\unity_0_cxx.cxx) [C:\arrow-build\src\arrow\arrow_shared.vcxproj]
      C:/arrow/cpp/src/arrow/array/array_nested.cc(660): note: see reference to function template instantiation 'arrow::Result<std::shared_ptr<arrow::Array>> arrow::`anonymous-namespace'::FlattenListViewArray<arrow::ListViewArray,false>(const ListViewArrayT &,arrow::MemoryPool *)' being compiled
              with
              [
                  ListViewArrayT=arrow::ListViewArray
              ] (compiling source file C:\arrow-build\src\arrow\CMakeFiles\arrow_shared.dir\Unity\unity_0_cxx.cxx)
      memory_pool.cc
    C:/arrow/cpp/src/arrow/array/array_nested.cc(291): error C2065: 'list_view_array_offset': undeclared identifier (compiling source file C:\arrow-build\src\arrow\CMakeFiles\arrow_shared.dir\Unity\unity_0_cxx.cxx) [C:\arrow-build\src\arrow\arrow_shared.vcxproj]

Use "if constexpr" in outer lambda.

cpp/src/arrow/array/array_nested.cc

@@ -267,7 +267,6 @@ template <typename ListViewArrayT, bool HasNulls>
 Result<std::shared_ptr<Array>> FlattenListViewArray(const ListViewArrayT& list_view_array,
  MemoryPool* memory_pool) {
  using offset_type = typename ListViewArrayT::offset_type;
- const int64_t list_view_array_offset = list_view_array.offset();
  const int64_t list_view_array_length = list_view_array.length();
  std::shared_ptr<arrow::Array> value_array = list_view_array.values();
 
@@ -282,18 +281,22 @@ Result<std::shared_ptr<Array>> FlattenListViewArray(const ListViewArrayT& list_v
  }
  }
 
- const auto* validity = list_view_array.data()->template GetValues<uint8_t>(0, 0);
  const auto* offsets = list_view_array.data()->template GetValues<offset_type>(1);
  const auto* sizes = list_view_array.data()->template GetValues<offset_type>(2);
 
- auto is_null_or_empty = [&](int64_t i) {
- if constexpr (HasNulls) {
+ std::function<bool(int64_t)> is_null_or_empty;
+ if constexpr (HasNulls) {
+ const auto* validity = list_view_array.data()->template GetValues<uint8_t>(0, 0);
+ const int64_t list_view_array_offset = list_view_array.offset();
+ is_null_or_empty = [&](int64_t i) {
  if (!bit_util::GetBit(validity, list_view_array_offset + i)) {
  return true;
  }
- }
- return sizes[i] == 0;
- };
+ return sizes[i] == 0;
+ };
+ } else {
+ is_null_or_empty = [&](int64_t i) { return sizes[i] == 0; };
+ }
 
  // Index of the first valid, non-empty list-view.
  int64_t first_i = 0;