CodeGen: minor cleanup, NFC
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@275241 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang