[clang] [CIR] Unblock simple C++ structure support (PR #138368)

[Erich Keane via cfe-commits]

================
@@ -365,10 +365,15 @@ mlir::Attribute ConstantEmitter::tryEmitPrivateForVarInit(const VarDecl &d) {
   if (!d.hasLocalStorage()) {
     QualType ty = cgm.getASTContext().getBaseElementType(d.getType());
     if (ty->isRecordType())
-      if (d.getInit() && isa<CXXConstructExpr>(d.getInit())) {
-        cgm.errorNYI(d.getInit()->getBeginLoc(),
-                     "tryEmitPrivateForVarInit CXXConstructExpr");
-        return {};
+      if (const CXXConstructExpr *e =
+              dyn_cast_or_null<CXXConstructExpr>(d.getInit())) {
+        const CXXConstructorDecl *cd = e->getConstructor();
----------------
erichkeane wrote:

Does classic code-gen assume that `cd` is non-null here?  I'm not positive it always is.

Also, do we/should we model elidable here?  

https://github.com/llvm/llvm-project/pull/138368