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/symbolstorageinterface.h

@@ -44,7 +44,6 @@ class SymbolStorageInterface
 {
 public:
     SymbolStorageInterface() = default;
-    virtual ~SymbolStorageInterface();
     SymbolStorageInterface(const SymbolStorageInterface &) = delete;
     SymbolStorageInterface &operator=(const SymbolStorageInterface &) = delete;
 
@@ -65,6 +64,9 @@ public:
     virtual Utils::optional<ProjectPartArtefact> fetchProjectPartArtefact(Utils::SmallStringView projectPartName) const = 0;
     virtual long long fetchLowestLastModifiedTime(FilePathId sourceId) const = 0;
     virtual Utils::optional<ProjectPartPch> fetchPrecompiledHeader(int projectPartId) const = 0;
+
+protected:
+    ~SymbolStorageInterface() = default;
 };
 
 } // namespace ClangBackEnd