[folly] Use allocateOverAligned to allocate over-aligned ConcurrentHa…

…shMap nodes

Summary:
ConcurrentHashMap uses alignment to ensure that there is sufficient
paddint to avoid false sharing, but doesn't ensure that the alignment
condition is actually met.  This causes problems under UBSAN.

Test Plan:
* existing unit tests
* no UBSAN failures

folly/concurrency/ConcurrentHashMap.h

@@ -156,6 +156,7 @@ class ConcurrentHashMap {
   using EnableHeterogeneousFind = std::enable_if_t<
       detail::EligibleForHeterogeneousFind<KeyType, HashFn, KeyEqual, K>::value,
       T>;
+  using SegAlloc = typename std::allocator_traits<Allocator>::template rebind_alloc<SegmentT>;
 
   float load_factor_ = SegmentT::kDefaultLoadFactor;
 
@@ -235,7 +236,7 @@ class ConcurrentHashMap {
       auto seg = segments_[i].load(std::memory_order_relaxed);
       if (seg) {
         seg->~SegmentT();
-        Allocator().deallocate((uint8_t*)seg, sizeof(SegmentT));
+        deallocateOverAligned(SegAlloc(), seg, 1);
       }
       segments_[i].store(
           o.segments_[i].load(std::memory_order_relaxed),
@@ -263,7 +264,7 @@ class ConcurrentHashMap {
       auto seg = segments_[i].load(std::memory_order_relaxed);
       if (seg) {
         seg->~SegmentT();
-        Allocator().deallocate((uint8_t*)seg, sizeof(SegmentT));
+        deallocateOverAligned(SegAlloc(), seg, 1);
       }
     }
     cohort_shutdown_cleanup();
@@ -727,13 +728,13 @@ class ConcurrentHashMap {
     SegmentT* seg = segments_[i].load(std::memory_order_acquire);
     if (!seg) {
       auto b = ensureCohort();
-      SegmentT* newseg = (SegmentT*)Allocator().allocate(sizeof(SegmentT));
+      SegmentT* newseg = allocateOverAligned(SegAlloc(), 1);
       newseg = new (newseg)
           SegmentT(size_ >> ShardBits, load_factor_, max_size_ >> ShardBits, b);
       if (!segments_[i].compare_exchange_strong(seg, newseg)) {
         // seg is updated with new value, delete ours.
         newseg->~SegmentT();
-        Allocator().deallocate((uint8_t*)newseg, sizeof(SegmentT));
+        deallocateOverAligned(SegAlloc(), newseg, 1);
       } else {
         seg = newseg;
         updateBeginAndEndSegments(i);