PR c++/89576 - if constexpr of lambda capture.

PR c++/89576 - if constexpr of lambda capture.

Now that we're doing implicit lambda capture in templates, we see x here as
the lambda capture.  maybe_convert_cond was doing nothing in a template, so
we never called mark_rvalue_use on x.  As part of the broad move toward
doing more processing of non-dependent expressions, let's do this
conversion.

	* semantics.c (maybe_convert_cond): Do convert a non-dependent
	condition in a template.
	* typeck.c (condition_conversion): Handle being called in a
	template.

From-SVN: r269433