Extirpate FOLLY_NODISCARD in folly/algorithm/simd/detail/UnrollUtils.h +5

Summary:
`[[nodiscard]]` is now available in all of our compilers. This diff changes the code to rely on it directly.

Generated with
```
reset && xbgs -l FOLLY_NODISCARD | grep -v Portability.h | grep -v json | grep -v xplat | grep -v ThirdParty/ | grep -v third-party | perl -pe 's/^fbsource.//' | xargs -n 100 perl -pe 's/FOLLY_NODISCARD/[[nodiscard]]/' -i
```

Reviewed By: ilvokhin

Differential Revision: D87101286

fbshipit-source-id: 0bdd69acfad34c583b15efbe22abbc95096305af

Author: r-barnes

folly/algorithm/simd/detail/UnrollUtils.h

@@ -42,7 +42,7 @@ struct UnrollUtils {
    * return an array of results.
    */
   template <typename T, std::size_t N, typename Op>
-  FOLLY_NODISCARD FOLLY_ALWAYS_INLINE static constexpr auto arrayMap(
+  [[nodiscard]] FOLLY_ALWAYS_INLINE static constexpr auto arrayMap(
       const std::array<T, N>& x, Op op) {
     return arrayMapImpl(x, op, std::make_index_sequence<N>());
   }
@@ -56,7 +56,7 @@ struct UnrollUtils {
    * (a + b) + (c + d)
    */
   template <typename T, std::size_t N, typename Op>
-  FOLLY_NODISCARD FOLLY_ALWAYS_INLINE static constexpr T arrayReduce(
+  [[nodiscard]] FOLLY_ALWAYS_INLINE static constexpr T arrayReduce(
       const std::array<T, N>& x, Op op) {
     return arrayReduceImpl<0, N>(x, op);
   }

folly/concurrency/CacheLocality.h

@@ -474,7 +474,7 @@ class CoreAllocator : private std::allocator<T> {
   };
 };
 
-class FOLLY_NODISCARD CoreAllocatorGuard {
+class [[nodiscard]] CoreAllocatorGuard {
  public:
   CoreAllocatorGuard(size_t numStripes, size_t stripe);
   ~CoreAllocatorGuard();

folly/coro/AsyncGenerator.h

@@ -211,7 +211,7 @@ template <
     typename Reference,
     typename Value = remove_cvref_t<Reference>,
     bool RequiresCleanup = false>
-class FOLLY_NODISCARD AsyncGenerator {
+class [[nodiscard]] AsyncGenerator {
   static_assert(
       std::is_constructible<Value, Reference>::value,
       "AsyncGenerator 'value_type' must be constructible from a 'reference'.");
@@ -253,7 +253,7 @@ class FOLLY_NODISCARD AsyncGenerator {
 
   class CleanupSemiAwaitable;
 
-  class FOLLY_NODISCARD CleanupAwaitable {
+  class [[nodiscard]] CleanupAwaitable {
    public:
     bool await_ready() noexcept { return !scopeExit_; }
 
@@ -292,7 +292,7 @@ class FOLLY_NODISCARD AsyncGenerator {
     folly::Executor::KeepAlive<> executor_;
   };
 
-  class FOLLY_NODISCARD CleanupSemiAwaitable {
+  class [[nodiscard]] CleanupSemiAwaitable {
    public:
     CleanupAwaitable viaIfAsync(Executor::KeepAlive<> executor) noexcept {
       return CleanupAwaitable{scopeExit_, std::move(executor)};

folly/coro/Noexcept.h

@@ -214,7 +214,7 @@ using as_noexcept_with_executor_base = TaskWithExecutorWrapperCrtp<
 } // namespace detail
 
 template <typename Inner, auto CancelCfg = OnCancel<void>{}>
-class FOLLY_NODISCARD as_noexcept_with_executor final
+class [[nodiscard]] as_noexcept_with_executor final
     : public detail::as_noexcept_with_executor_base<Inner, CancelCfg> {
  protected:
   using detail::as_noexcept_with_executor_base<Inner, CancelCfg>::

folly/coro/SharedLock.h

@@ -36,7 +36,7 @@ namespace coro {
 namespace detail {
 
 template <typename Mutex, typename Policy>
-class FOLLY_NODISCARD LockBase {
+class [[nodiscard]] LockBase {
  public:
   static_assert(std::is_same_v<
                 bool,