Fix -Wimplicit-fallthrough warning

Reviewers: EricWF, ldionne, mclow.lists

Reviewed By: ldionne

Subscribers: christof, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54234

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@346369 91177308-0d34-0410-b5e6-96231b3b80d8

include/utility

@@ -989,8 +989,10 @@ __murmur2_or_cityhash<_Size, 32>::operator()(const void* __key, _Size __len)
     {
     case 3:
         __h ^= __data[2] << 16;
+        _LIBCPP_FALLTHROUGH();
     case 2:
         __h ^= __data[1] << 8;
+        _LIBCPP_FALLTHROUGH();
     case 1:
         __h ^= __data[0];
         __h *= __m;