[clang] [clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854) (PR #96301)

[Richard Smith via cfe-commits]

================
@@ -1392,7 +1392,8 @@ class CXXRecordDecl : public RecordDecl {
   bool allowConstDefaultInit() const {
     return !data().HasUninitializedFields ||
            !(data().HasDefaultedDefaultConstructor ||
-             needsImplicitDefaultConstructor());
+             needsImplicitDefaultConstructor()) ||
+           hasInClassInitializer();
----------------
zygoloid wrote:

What happens for:

```
struct A {
  union {
    struct {
      int n = 5;
      int m;
    };
  };
};
const A a;
```

This is a language extension so we get to choose its semantics, but I think the 
most logical choice is to reject because the union member is not *fully* 
initialized.

https://github.com/llvm/llvm-project/pull/96301
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