[CodeGen] Set construction vtable visibility after creating initializer

We must only set the construction vtable visibility after we create the
vtable initializer, otherwise the global value will be treated as
declaration rather than definition and the visibility won't be set.

Differential Revision: https://reviews.llvm.org/D58010

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353742 91177308-0d34-0410-b5e6-96231b3b80d8