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/tools/clangrefactoringbackend/source/symbolindexinginterface.h

@@ -33,8 +33,15 @@ namespace ClangBackEnd {
 class SymbolIndexingInterface
 {
 public:
+    SymbolIndexingInterface() = default;
+    SymbolIndexingInterface(const SymbolIndexingInterface&) = delete;
+    SymbolIndexingInterface &operator=(const SymbolIndexingInterface&) = delete;
+
     virtual void updateProjectParts(V2::ProjectPartContainers &&projectParts,
                                     V2::FileContainers &&generatedFiles) = 0;
+
+protected:
+    ~SymbolIndexingInterface() = default;
 };
 
 } // namespace ClangBackEnd