[region-isolation] Enable region isolation by default with strict-concurrency.

I added a disable flag -disable-region-based-isolation-with-strict-concurrency
so that we do not need to update the current tests. It is only available when
asserts are enabled to ensure users cannot use it.

rdar://125918028

lib/Frontend/CompilerInvocation.cpp

@@ -1060,6 +1060,16 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (Opts.StrictConcurrencyLevel == StrictConcurrency::Complete) {
     Opts.enableFeature(Feature::IsolatedDefaultValues);
     Opts.enableFeature(Feature::GlobalConcurrency);
+
+    // If asserts are enabled, allow for region based isolation to be disabled
+    // with a flag. This is intended only to be used with tests.
+    bool enableRegionIsolation = true;
+#ifndef NDEBUG
+    enableRegionIsolation =
+        !Args.hasArg(OPT_disable_strict_concurrency_region_based_isolation);
+#endif
+    if (enableRegionIsolation)
+      Opts.enableFeature(Feature::RegionBasedIsolation);
   }
 
   Opts.WarnImplicitOverrides =