Revert "remove unused SvRefBase constructor"

This reverts commit 09a90c2ff5d39c3ae61b4041c3b39d3da0c640c1.

Apparently causes a crash in CppunitTest_sc_macros_test; it is
suspicious that the copy ctor is not made private in this commit, since
the compiler generated one would do something different.

Change-Id: Icb1121332d83e9f08d89535e3da6e10d690ac88a
crashrep: loplugin:unreffun

Change-Id: I83d4c3ee02e59d40e793288d705c3fd06ebe3219