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

[Aaron Ballman via cfe-commits]

================
@@ -2148,6 +2161,35 @@ void CXXRecordDecl::completeDefinition(CXXFinalOverriderMap *FinalOverriders) {
   for (conversion_iterator I = conversion_begin(), E = conversion_end();
        I != E; ++I)
     I.setAccess((*I)->getAccess());
+
+  ASTContext &Context = getASTContext();
+
+  if (isAggregate() && hasUserDeclaredConstructor() &&
+      !Context.getLangOpts().CPlusPlus20) {
+    // Diagnose any aggregate behavior changes in C++20
+    for (field_iterator I = field_begin(), E = field_end(); I != E; ++I) {
+      if (const auto *attr = I->getAttr<ExplicitInitAttr>()) {
----------------
AaronBallman wrote:

```suggestion
      if (const auto *AT = I->getAttr<ExplicitInitAttr>()) {
```
(`attr` doesn't match the naming conventions, but `Attr` is a type name, so just picked a name that met the usual coding style, feel free to rename to something better if you'd prefer.)

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