PR c++/92871 - bad code with xvalue and GNU ?: extension.

PR c++/92871 - bad code with xvalue and GNU ?: extension.

I steered Jakub wrong on the desired behavior for temp-extend1.C in the
context of bug 92831; it doesn't make sense to try to extend the lifetime of
a temporary that we've already materialized to evaluate the test.  So this
patch munges the stabilized expression so that it won't be subject to
lifetime extension.

	* call.c (prevent_lifetime_extension): New.
	(build_conditional_expr_1): Use it.