Added an assert in `isConstantInitializer`: initializer lists must be in semantic form

[gribozavr]

@rsmith Ping?

clang/lib/AST/Expr.cpp

bool Expr::isConstantInitializer(ASTContext &Ctx, bool IsForRef,
   }
   case DesignatedInitUpdateExprClass: {
     const DesignatedInitUpdateExpr *DIUE = cast<DesignatedInitUpdateExpr>(this);
     return DIUE->getBase()->isConstantInitializer(Ctx, false, Culprit) &&
            DIUE->getUpdater()->isConstantInitializer(Ctx, false, Culprit);
   }
   case InitListExprClass: {
     const InitListExpr *ILE = cast<InitListExpr>(this);
+    assert(ILE->isSemanticForm() && "InitListExpr must be in semantic form");
     if (ILE->getType()->isArrayType()) {
       unsigned numInits = ILE->getNumInits();
       for (unsigned i = 0; i < numInits; i++) {
         if (!ILE->getInit(i)->isConstantInitializer(Ctx, false, Culprit))
           return false;
       }
       return true;
     }