[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/Frontend/CompilerInvocation.cpp

@@ -1184,6 +1184,12 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                    Target.isOSDarwin());
   Opts.EnableSILOpaqueValues |= Args.hasArg(OPT_enable_sil_opaque_values);
 
+#if SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
+  Opts.UseDarwinPreStableABIBit = false;
+#else
+  Opts.UseDarwinPreStableABIBit = true;
+#endif
+
   // Must be processed after any other language options that could affect
   // platform conditions.
   bool UnsupportedOS, UnsupportedArch;