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

[Erich Keane via cfe-commits]

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@@ -1419,6 +1419,35 @@ is not specified.
   }];
 }
 
+def ExplicitInitDocs : Documentation {
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erichkeane wrote:

This is a little lacking for me?  Semantics, limitations, examples of how this is intended to be used, warnings about 'make sure you own the type' sorta thing would be appreciated.

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