PR c++/55442 - memory-hog with highly recursive constexpr.

PR c++/55442 - memory-hog with highly recursive constexpr.

This testcase in the PR is extremely recursive, and therefore uses a huge
amount of memory on caching the results of individual calls.  We no longer
need to track all calls to catch infinite recursion, as we have other limits
on maximum depth and operations count.  So let's only cache a few calls at
the top level: 8 seems to be a reasonable compromise.

gcc/c-family/
	* c.opt (fconstexpr-loop-limit): New.
gcc/cp/
	* constexpr.c (push_cx_call_context): Return depth.
	(cxx_eval_call_expression): Don't cache past constexpr_cache_depth.

From-SVN: r272765