[Clang] Fix for Tighten restrictions on enum out of range diagnostic

Ok it looks like this is a bit more subtle, I broke the llvm-test-suite file
paq8p.cpp again. We need both conditions to be true Info.EvalMode ==
EvalInfo::EM_ConstantExpression && Info.InConstantContext. We need to be in a
context that requires a constant value but also in a constant expression context.

Differential Revision: https://reviews.llvm.org/D131704

clang/lib/AST/ExprConstant.cpp

@@ -13534,6 +13534,7 @@ bool IntExprEvaluator::VisitCastExpr(const CastExpr *E) {
     }
 
     if (Info.Ctx.getLangOpts().CPlusPlus && Info.InConstantContext &&
+        Info.EvalMode == EvalInfo::EM_ConstantExpression &&
         DestType->isEnumeralType()) {
       const EnumType *ET = dyn_cast<EnumType>(DestType.getCanonicalType());
       const EnumDecl *ED = ET->getDecl();