Work around a GCC 4.7-specific issue: due to implementing older rules for

implicit declarations of move operations, GCC 4.7 would find that SelectPiece
has neither a move constructor nor a copy constructor. The copy constructor was
(correctly) deleted because the class has a member of move-only type, and the
move constructor was (incorrectly, per current C++ rules) not provided because
the class has a copy-only base class (in turn because it explicitly declares a
destructor).

llvm-svn: 281363