[clang] Short-circuit evaluation in ::EvaluateAsConstantExpr

Use FastEvaluateAsRValue() in EvaluateAsConstantExpr() as well, to
short-circuit evaluation of simple integrals.

Differential Revision: https://reviews.llvm.org/D138115

clang/lib/AST/ExprConstant.cpp

@@ -15051,6 +15051,12 @@ static bool FastEvaluateAsRValue(const Expr *Exp, Expr::EvalResult &Result,
     return true;
   }
 
+  if (const auto *L = dyn_cast<CXXBoolLiteralExpr>(Exp)) {
+    Result.Val = APValue(APSInt(APInt(1, L->getValue())));
+    IsConst = true;
+    return true;
+  }
+
   // This case should be rare, but we need to check it before we check on
   // the type below.
   if (Exp->getType().isNull()) {
@@ -15235,6 +15241,9 @@ bool Expr::EvaluateAsConstantExpr(EvalResult &Result, const ASTContext &Ctx,
                                   ConstantExprKind Kind) const {
   assert(!isValueDependent() &&
          "Expression evaluator can't be called on a dependent expression.");
+  bool IsConst;
+  if (FastEvaluateAsRValue(this, Result, Ctx, IsConst))
+    return true;
 
   ExprTimeTraceScope TimeScope(this, Ctx, "EvaluateAsConstantExpr");
   EvalInfo::EvaluationMode EM = EvalInfo::EM_ConstantExpression;