[clang] [Clang] Implement CWG2598: Union of non-literal types (PR #78195)

[Erich Keane via cfe-commits]

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@@ -1389,7 +1389,11 @@ bool CXXRecordDecl::isLiteral() const {
                              : hasTrivialDestructor()))
     return false;
 
-  if (isLambda() && !LangOpts.CPlusPlus17)
+  // Lambdas are literal types since C++17.
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erichkeane wrote:

So why is `isAggregate` or `hasConstexprNonCopyMoveConstructor`/etc not just `return true` here?  Am I missing something? 

https://github.com/llvm/llvm-project/pull/78195