Silence -Wdeprecated warning on clang

[HowardHinnant]

• definition of implicit copy constructor for 'SnappySinkAllocator'
  is deprecated because it has a user-declared destructor.

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[HowardHinnant]

I signed it!

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CLAs look good, thanks!

[cmumford]

@googlebot rescan

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[Quuxplusone]

This warning is still happening in main. I confirm that Howard's fix is the appropriate fix.

snappy.cc:2344:3: error: definition of implicit copy constructor for 'SnappySinkAllocator' is deprecated
because it has a user-provided destructor [-Werror,-Wdeprecated-copy-with-user-provided-dtor]
 2344 |   ~SnappySinkAllocator() {}
      |   ^
snappy.cc:2194:9: note: in implicit copy constructor for 'snappy::SnappySinkAllocator'
first required here
 2194 |       : allocator_(allocator),
      |         ^
snappy.cc:2391:46: note: in instantiation of member function
'snappy::SnappyScatteredWriter<snappy::SnappySinkAllocator>::SnappyScatteredWriter' requested here
 2391 |   SnappyScatteredWriter<SnappySinkAllocator> writer(allocator);
      |                                              ^