Add an example showing the alternative to nested designators

[shafik]

Summary

We have an example showing that a nested designators are a c99 extension but not an example showing how to achieve the same thing using a brace-init-list in a designated-initializer-clause.

clang/test/SemaCXX/cxx2a-initializer-aggregates.cpp

 struct A { int x, y; };
 struct B { A a; };
 
 A a1 = {
   .y = 1, // reorder-note {{previous initialization for field 'y' is here}}
   .x = 2 // reorder-error {{ISO C++ requires field designators to be specified in declaration order; field 'y' will be initialized after field 'x'}}
 };
 int arr[3] = {[1] = 5}; // pedantic-error {{array designators are a C99 extension}}
-B b = {.a.x = 0}; // pedantic-error {{nested designators are a C99 extension}}
+B b1 = {.a.x = 0}; // pedantic-error {{nested designators are a C99 extension}}
+B b2 = {.a = {.x = 0, .y = 1}}; // ok, we don't need nested designators to initialize the members of A
 A a2 = {
   .x = 1, // pedantic-error {{mixture of designated and non-designated initializers in the same initializer list is a C99 extension}}
   2 // pedantic-note {{first non-designated initializer is here}}
 };
 A a3 = {
   1, // pedantic-note {{first non-designated initializer is here}}
   .y = 2 // pedantic-error {{mixture of designated and non-designated initializers in the same initializer list is a C99 extension}}
 };