AST: Add ModuleDecl::isResilient()

This cleans up some code. I'm keeping the ResilienceStrategy enum around
though, in case we want to use it to version the ABI in the future.

lib/Serialization/ModuleFile.cpp

@@ -1455,7 +1455,7 @@ Status ModuleFile::associateWithFileContext(FileUnit *file,
     return error(Status::TargetIncompatible);
   }
   if (ctx.LangOpts.EnableTargetOSChecking &&
-      M->getResilienceStrategy() != ResilienceStrategy::Resilient &&
+      !M->isResilient() &&
       isTargetTooNew(moduleTarget, ctx.LangOpts.Target)) {
     return error(Status::TargetTooNew);
   }