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/clangrefactoringbackend-source.pri

@@ -67,6 +67,5 @@ SOURCES += \
     $$PWD/sourcerangefilter.cpp \
     $$PWD/symbolindexer.cpp \
     $$PWD/symbolentry.cpp \
-    $$PWD/symbolstorageinterface.cpp \
     $$PWD/projectpartartefact.cpp \
     $$PWD/filestatuscache.cpp