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/plugins/clangrefactoring/projectpartproviderinterface.h

@@ -39,7 +39,6 @@ class ProjectPartProviderInterface
 {
 public:
     ProjectPartProviderInterface() = default;
-    virtual ~ProjectPartProviderInterface();
 
     ProjectPartProviderInterface(const ProjectPartProviderInterface&) = delete;
     ProjectPartProviderInterface& operator=(const ProjectPartProviderInterface&) = delete;
@@ -49,6 +48,9 @@ public:
 
     virtual CppTools::ProjectPart *projectPart(const QString &projectPartId) const = 0;
     virtual ClangBackEnd::V2::FileContainers generatedFiles() const = 0;
+
+protected:
+    ~ProjectPartProviderInterface() = default;
 };
 
 } // namespace ClangRefactoring