SemaInit.cpp: Fix a warning with -Asserts. [-Wunused-variable]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213345 91177308-0d34-0410-b5e6-96231b3b80d8
Mark the vtable used when defining implicit copy and move ctors

I don't think other implicit members like copy assignment and move
assignment require this treatment, because they should already be
operating on a constructed object.

Fixes PR20351.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213346 91177308-0d34-0410-b5e6-96231b3b80d8