Don't try to mark virtual members referenced for classes where the key function

Don't try to mark virtual members referenced for classes where the key function
is not defined in the current translation unit. Doing so lead to compile errors
such as PR9114.

Instead, when CodeGen is building the vtable, don't try to emit a definition
for functions that aren't marked used in the current translation unit.
Fixes PR9114.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124768 91177308-0d34-0410-b5e6-96231b3b80d8