[clang] Add Clang attribute to ensure that fields are initialized explicitly (PR #102040)

[Erich Keane via cfe-commits]

================
@@ -1472,3 +1472,56 @@ template<typename T> struct Outer {
   };
 };
 Outer<int>::Inner outerinner;
+
+void aggregate() {
+  struct NonAgg {
+    NonAgg() { }
+    [[clang::requires_explicit_initialization]] int f;  // expected-warning {{attribute is ignored}}
+  };
+  NonAgg nonagg;
+  (void)nonagg;
+
+  struct S {
+    [[clang::requires_explicit_initialization]] int x; // expected-note {{declared}} // expected-note {{declared}} // expected-note {{declared}} // expected-note {{declared}}
----------------
erichkeane wrote:

```suggestion
    [[clang::requires_explicit_initialization]] int x; // expected-note 4{{declared here}}
```

THOUGH, I VASTLY prefer to use 'bookmarks' for diagnostics and have these notes be shown in text right below the warning.

Also, please don't use partial matches so aggressively, it makes reviewing these now/in the future much more difficult

https://github.com/llvm/llvm-project/pull/102040