[IRGen][runtime] Prepare to change the is-Swift bit in class metadata. (#13595)

Swift class metadata has a bit to distinguish it from non-Swift Objective-C
classes. The stable ABI will use a different bit so that stable Swift and
pre-stable Swift can be distinguished from each other.

No bits are actually changed yet. Enabling the new bit needs to wait for
other coordination such as libobjc.

rdar://35767811

lib/IRGen/IRGenModule.cpp

@@ -134,6 +134,7 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
       OutputFilename(OutputFilename),
       TargetInfo(SwiftTargetInfo::get(*this)), DebugInfo(nullptr),
       ModuleHash(nullptr), ObjCInterop(Context.LangOpts.EnableObjCInterop),
+      UseDarwinPreStableABIBit(Context.LangOpts.UseDarwinPreStableABIBit),
       Types(*new TypeConverter(*this)) {
   irgen.addGenModule(SF, this);