Add a visitor for lambda expressions to RecordExprEvaluator in ExprConstant.cpp that creates an empty APValue of Struct type to represent the closure object. Additionally, add a LambdaExpr visitor to the TemporaryExprEvaluator that forwards constant evaluation of immediately-called-lambda-expressions to the one in RecordExprEvaluator through VisitConstructExpr.

This patch supports:
constexpr auto ID = [] (auto a) { return a; };
static_assert(ID(3.14) == 3.14);
static_assert([](auto a) { return a + 1; }(10) == 11);

Lambda captures are still not supported for constexpr lambdas.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@291416 91177308-0d34-0410-b5e6-96231b3b80d8