[clang-reorder-fields] Support designated initializers

Initializer lists with designators, missing elements or omitted braces can now be rewritten. Any missing designators are added and they get sorted according to the new order.

```
struct Foo {
  int a;
  int b;
  int c;
};
struct Foo foo = { .a = 1, 2, 3 }
```

when reordering elements to "b,a,c" becomes:

```
struct Foo {
  int b;
  int a;
  int c;
};
struct Foo foo = { .b = 2, .a = 1, .c = 3 }
```

Author: vvuksanovic

clang-tools-extra/clang-reorder-fields/CMakeLists.txt

@@ -4,6 +4,7 @@ set(LLVM_LINK_COMPONENTS
 )
 
 add_clang_library(clangReorderFields STATIC
+  utils/Designator.cpp
   ReorderFieldsAction.cpp
 
   DEPENDS