Frontend: Introduce an alias for the IsolatedAny experimental feature.

To preserve compatibility with older compilers that do not allow `IsolatedAny`
to be enabled in production compilers, use an alias experimental feature when
building the stdlib (`IsolatedAny2`).

Also, add `@_allowFeatureSuppression(IsolatedAny)` in a couple spots it was
forgotten.

Partially resolves rdar://125138945

lib/Frontend/CompilerInvocation.cpp

@@ -886,6 +886,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 
       if (*feature == Feature::NoncopyableGenerics2)
         Opts.enableFeature(Feature::NoncopyableGenerics);
+
+      if (*feature == Feature::IsolatedAny2)
+        Opts.enableFeature(Feature::IsolatedAny);
     }
 
     // Hack: In order to support using availability macros in SPM packages, we