Allow value generics within the stdlib

Value Generics require support from libswiftCore, but that
means it is always okay to use value generics within libswiftCore
itself, since the runtime support is necessarily present.

Author: tbkka

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -568,8 +568,10 @@ static void checkGenericParams(GenericContext *ownerCtx) {
     if (gp->isValue()) {
       // Value generic nominal types require runtime support.
       //
+      // Stdlib/libswiftCore carries support for this feature
       // Embedded doesn't require runtime support for this feature.
       if (isa<NominalTypeDecl>(decl) &&
+          !decl->getModuleContext()->isStdlibModule() &&
           !ctx.LangOpts.hasFeature(Feature::Embedded)) {
         TypeChecker::checkAvailability(
           gp->getSourceRange(),