AST: Rename GenericContext::isGeneric to hasGenericParamList

`isGeneric` is a misleading name because this method checks for the
existence of a `GenericParamList`, which is not implied by genericity.

lib/AST/DeclContext.cpp

@@ -629,7 +629,7 @@ swift::FragileFunctionKindRequest::evaluate(Evaluator &evaluator,
 bool DeclContext::isInnermostContextGeneric() const {
   if (auto Decl = getAsDecl())
     if (auto GC = Decl->getAsGenericContext())
-      return GC->isGeneric();
+      return GC->hasGenericParamList();
   return false;
 }