Clang: Improve interfaces

The interfaces should never used to handle ownership. So it is now using
protected destructors. Copy operations are forbidden too.

https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#c35-a-base-class-destructor-should-be-either-public-and-virtual-or-protected-and-nonvirtual

Change-Id: Ib0b60a73a7ec130973b5cb0095cc5b2f10fa0758
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/libs/clangsupport/clangcodemodelclientinterface.h

@@ -63,6 +63,9 @@ public:
     virtual void references(const ReferencesMessage &message) = 0;
     virtual void followSymbol(const FollowSymbolMessage &message) = 0;
     virtual void tooltip(const ToolTipMessage &message) = 0;
+
+protected:
+    ~ClangCodeModelClientInterface() = default;
 };
 
 } // namespace ClangBackEnd