[Serialization] Add flag to force unsafe recovery from some xref failures

Intro a deserialization mode controlled by the flag
`-experimental-force-workaround-broken-modules` to attempt unsafe
recovery from deserialization failures caused by project issues.

The one issue handled at this time is when a type moves from one module
to another. With this new mode the compiler may be able to pick a
matching type in a different module. This is risky to use, but may help
in a pinch for a client to fix and issue in a library over which they
have no control.

lib/Frontend/CompilerInvocation.cpp

@@ -564,6 +564,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.EnableDeserializationSafety
       = A->getOption().matches(OPT_enable_deserialization_safety);
   }
+  Opts.ForceWorkaroundBrokenModules
+    |= Args.hasArg(OPT_force_workaround_broken_modules);
 
   // Whether '/.../' regex literals are enabled. This implies experimental
   // string processing.