Make sure we query hasInitAccessor before looking for the init accessor.

This is a workaround for the lack of a request for getting accessors.

Author: DougGregor

lib/AST/Decl.cpp

@@ -7187,8 +7187,10 @@ bool VarDecl::isMemberwiseInitialized(bool preferDeclaredProperties) const {
   // If this is a computed property with `init` accessor, it's
   // memberwise initializable when it could be used to initialize
   // other stored properties.
-  if (auto *init = getAccessor(AccessorKind::Init))
-    return init->getAttrs().hasAttribute<InitializesAttr>();
+  if (hasInitAccessor()) {
+    if (auto *init = getAccessor(AccessorKind::Init))
+      return init->getAttrs().hasAttribute<InitializesAttr>();
+  }
 
   // If this is a computed property, it's not memberwise initialized unless
   // the caller has asked for the declared properties and it is either a