When we refer to a captured variable from a constant-expression context
inside a lambda, the closure (like any function parameter) is not constant
because we aren't in a call, so we don't have an argument.  So the capture
is non-constant.  But if the captured variable is constant, we might be able
to use it directly in constexpr evaluation.

	PR c++/82643
	PR c++/87327
	* constexpr.c (cxx_eval_constant_expression): In a lambda function,
	try evaluating the captured variable directly.

From-SVN: r269951