Annotate absl::FixedArray to warn when unused.

PiperOrigin-RevId: 715803554
Change-Id: Ic0b733641174922b89f4d8c3ae2cb2762db76d5c

Author: ckennelly

absl/container/fixed_array.h

@@ -41,6 +41,7 @@
 #include <type_traits>
 
 #include "absl/algorithm/algorithm.h"
+#include "absl/base/attributes.h"
 #include "absl/base/config.h"
 #include "absl/base/dynamic_annotations.h"
 #include "absl/base/internal/throw_delegate.h"
@@ -74,7 +75,7 @@ constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
 // `std::vector`.
 template <typename T, size_t N = kFixedArrayUseDefault,
           typename A = std::allocator<T>>
-class FixedArray {
+class ABSL_ATTRIBUTE_WARN_UNUSED FixedArray {
   static_assert(!std::is_array<T>::value || std::extent<T>::value > 0,
                 "Arrays with unknown bounds cannot be used with FixedArray.");