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

[Erich Keane via cfe-commits]

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@@ -1861,6 +1861,13 @@ def Leaf : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def ExplicitInit : InheritableAttr {
+  let Spellings = [Clang<"requires_explicit_initialization", 1>];
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erichkeane wrote:

```suggestion
  let Spellings = [Clang<"requires_explicit_initialization">];
```

Its the default value

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