Solve clang-tidy warnings

[benrubson]

See Travis CI log on master branch, the clang build, to list clang-tidy warnings to correct.

[benrubson]

May I ask some help to correctly solve the following warnings ?
@rfjakob, do you think you could handle them ?
Will make the build log even more clean.
Thank you very much 👍

DirNode.cpp:133:7: warning: class 'RenameOp' defines a destructor and a copy constructor but does not define a copy assignment operator, a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]
class RenameOp {
      ^
NullCipher.cpp:50:7: warning: class 'NullKey' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]
class NullKey : public AbstractCipherKey {
      ^
NullCipher.cpp:56:7: warning: class 'NullDestructor' defines a destructor, a copy constructor and a copy assignment operator but does not define a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]
class NullDestructor {
      ^
SSL_Cipher.cpp:277:7: warning: class 'SSLKey' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]
class SSLKey : public AbstractCipherKey {
      ^
XmlReader.cpp:165:7: warning: class 'XmlNode' defines a destructor but does not define a copy constructor, a copy assignment operator, a move constructor or a move assignment operator [cppcoreguidelines-special-member-functions]
class XmlNode : virtual public XmlValue {
      ^

[benrubson]

OK, so just one Clang-tidy warning remaining, the tiny one above.
Any thoughts perhaps @vgough @rfjakob ?
We could simply disable this warning class, but perhaps it's simple and worth making the required definitions, as a best practice ?

Link to the rule :
http://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines-special-member-functions.html

[benrubson]

OK, last thing to solve before we could I think release 1.9.3.
Thx for your help 👍