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

[Aaron Ballman via cfe-commits]

================
@@ -1885,6 +1885,13 @@ def Leaf : InheritableAttr {
   let SimpleHandler = 1;
 }
 
+def ExplicitInit : InheritableAttr {
+  let Spellings = [Clang<"requires_explicit_initialization", 1>];
----------------
AaronBallman wrote:

```suggestion
  let Spellings = [Clang<"requires_explicit_initialization">];
```
No need for that, it's the default value anyway.

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