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/clangpchmanager/precompiledheaderstorageinterface.h

@@ -33,7 +33,6 @@ class PrecompiledHeaderStorageInterface
 {
 public:
     PrecompiledHeaderStorageInterface() = default;
-    virtual ~PrecompiledHeaderStorageInterface();
 
     PrecompiledHeaderStorageInterface(const PrecompiledHeaderStorageInterface&) = delete;
     PrecompiledHeaderStorageInterface &operator=(const PrecompiledHeaderStorageInterface&) = delete;
@@ -43,6 +42,9 @@ public:
                                          long long pchBuildTime) = 0;
 
     virtual void deletePrecompiledHeader(Utils::SmallStringView projectPartName) = 0;
+
+protected:
+    ~PrecompiledHeaderStorageInterface() = default;
 };
 
 } // namespace ClangPchManager