CodeGen: minor cleanup, NFC

Initialise more members in initializer lists.  Invert the condition that had
grown to be pretty confusing.  The `_objc_empty_vtable` is only used on macOS
<10.9.  This simplifies the code.  NFC.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@275241 91177308-0d34-0410-b5e6-96231b3b80d8