[AST] Introduce Decl::addAttribute

Introduce a convenience entrypoint that also calls `attachToDecl` on
the attribute, and migrate all existing uses of `getAttrs().add` onto
it.

lib/Frontend/ModuleInterfaceSupport.cpp

@@ -804,7 +804,7 @@ public:
     // the order in which previous implementations printed these attributes.
     for (auto attr = clonedAttrs.rbegin(), end = clonedAttrs.rend();
          attr != end; ++attr) {
-      extension->getAttrs().add(const_cast<DeclAttribute *>(*attr));
+      extension->addAttribute(const_cast<DeclAttribute *>(*attr));
     }
 
     ctx.evaluator.cacheOutput(ExtendedTypeRequest{extension},