c++: Simplify uses of LAMBDA_EXPR_EXTRA_SCOPE

I noticed there already exists a getter to get the scope of a lambda
from its type directly rather than needing to go via
CLASSTYPE_LAMBDA_EXPR, we may as well use it.

gcc/cp/ChangeLog:

	* module.cc (trees_out::get_merge_kind): Use
	LAMBDA_TYPE_EXTRA_SCOPE instead of LAMBDA_EXPR_EXTRA_SCOPE.
	(trees_out::key_mergeable): Likewise.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>