Update DigestBuilder to align its cpuLocalBuffers_

Summary:
CpuLocalBuffer need extend alignment, which isn't honored by std::vector's default allocator due to compiler bug

  http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3396.htm
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65122
  https://bugs.llvm.org/show_bug.cgi?id=22634

the fix is to use custom allocator to honor extend alignment requirement.

Reviewed By: yfeldblum

Differential Revision: D21405041

fbshipit-source-id: 53cfe1bfd66580990c8a1a0c5f0c09f6651bda24

folly/stats/detail/DigestBuilder.h

@@ -18,6 +18,7 @@
 
 #include <memory>
 
+#include <folly/Memory.h>
 #include <folly/SpinLock.h>
 
 namespace folly {
@@ -60,7 +61,13 @@ class DigestBuilder {
     std::unique_ptr<DigestT> digest;
   };
 
-  std::vector<CpuLocalBuffer> cpuLocalBuffers_;
+  //  cpulocalbuffer_alloc custom allocator is necessary until C++17
+  //    http://open-std.org/JTC1/SC22/WG21/docs/papers/2012/n3396.htm
+  //    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65122
+  //    https://bugs.llvm.org/show_bug.cgi?id=22634
+  using cpulocalbuffer_alloc =
+      AlignedSysAllocator<CpuLocalBuffer, FixedAlign<alignof(CpuLocalBuffer)>>;
+  std::vector<CpuLocalBuffer, cpulocalbuffer_alloc> cpuLocalBuffers_;
   size_t bufferSize_;
   size_t digestSize_;
 };