Frontend: Introduce an alias for the NoncopyableGenerics experimental feature.

The `.swiftinterface` of the standard library must remain compatible with some
older compilers. Unfortunately, some of those older compilers do not allow the
experimental feature `NoncopyableGenerics` to be enabled in production. To
allow the stdlib to build with non-copyable generics enabled and still have the
older compilers consume its interface, we have to use a new experimental
feature identifier that they do not know about.

Partially resolves rdar://125138945

lib/Frontend/CompilerInvocation.cpp

@@ -883,6 +883,9 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
 #else
       Opts.enableFeature(*feature);
 #endif
+
+      if (*feature == Feature::NoncopyableGenerics2)
+        Opts.enableFeature(Feature::NoncopyableGenerics);
     }
 
     // Hack: In order to support using availability macros in SPM packages, we