c++: trivial lambda pruning [PR120716]

c++: trivial lambda pruning [PR120716]

In this testcase there is nothing in the lambda except a static_assert which
mentions a variable from the enclosing scope but does not odr-use it, so we
want prune_lambda_captures to remove its capture.  Since the lambda is so
empty, there's nothing in the body except the DECL_EXPR of the capture
proxy, so pop_stmt_list moves that into the enclosing STATEMENT_LIST and
passes the 'body' STATEMENT_LIST to free_stmt_list.  As a result, passing
'body' to prune_lambda_captures is wrong; we should instead pass the
enclosing scope, i.e. cur_stmt_list.

	PR c++/120716

gcc/cp/ChangeLog:

	* lambda.cc (finish_lambda_function): Pass cur_stmt_list to
	prune_lambda_captures.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/lambda/lambda-constexpr3.C: New test.
	* g++.dg/cpp0x/lambda/lambda-constexpr3a.C: New test.