Reduce accumulated garbage in constexpr evaluation.

Reduce accumulated garbage in constexpr evaluation.

We want to evaluate the arguments to a call before looking into the cache so
that we have constant values, but if we then find the call in the cache we
end up with a TREE_LIST that we don't end up using; in highly recursive
constexpr evaluation this ends up being a large proportion of the garbage
generated.

The cxx_eval_increment_expression hunk is less important, but it's an easy
tweak; we only use the MODIFY_EXPR to evaluate it, so after that it's
garbage.

	* constexpr.c (cxx_eval_call_expression): ggc_free any bindings we
	don't save.
	(cxx_eval_increment_expression): ggc_free the MODIFY_EXPR after
	evaluating it.

From-SVN: r271909