[clang] [Clang] Implement P3074R7: trivial unions (C++26) (PR #185886)

[Corentin Jabot via cfe-commits]

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@@ -1180,7 +1180,11 @@ void CXXRecordDecl::addedMember(Decl *D) {
       // C++11 [class]p5:
       //   A default constructor is trivial if [...] no non-static data member
       //   of its class has a brace-or-equal-initializer.
-      data().HasTrivialSpecialMembers &= ~SMF_DefaultConstructor;
+      // P3074R7 [class.default.ctor]p3:
----------------
cor3ntin wrote:

```suggestion
      // C++26 [class.default.ctor]p3:
```

And everywhere else there is a similar comment

https://github.com/llvm/llvm-project/pull/185886