[clang] fix explicit incomplete enum (PR #184210)

[Erich Keane via cfe-commits]

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@@ -6464,6 +6464,9 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
   if (checkPlaceholderForOverload(S, From))
     return ExprError();
 
+  if (From->containsErrors())
+    return S.CreateRecoveryExpr(From->getBeginLoc(), From->getEndLoc(), {From}, T);
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erichkeane wrote:

I guess that is fair, I think an implicit cast makes sense too.  

https://github.com/llvm/llvm-project/pull/184210