[Clang] Change Attribute plugin to link clang-cpp like other examples (#109319)
Change the Attribute example plugin to use clang_target_link_libraries
instead of target_link_libraries so libclang-cpp is linked when the
CLANG_LINK_CLANG_DYLIB CMake option is used.
This change will allow building the plugin on windows when building llvm
and clang as a shared library with explicit visibility macros enabled.