PR c++/88752 - ICE with lambda and constexpr if.

PR c++/88752 - ICE with lambda and constexpr if.

In this testcase, we look for an instantiation of the outer lambda from
within the inner lambda.  enclosing_instantiation_of didn't handle this
properly, as it assumed that any references would be from the same lambda
nesting depth.  Fixed thus.

	* cp-tree.h (LAMBDA_EXPR_INSTANTIATED): New.
	* pt.c (tsubst_lambda_expr): Set it.
	(instantiated_lambda_fn_p): Check it.
	(enclosing_instantiation_of): Use it.

From-SVN: r268424