c++: Fix constexpr cleanup error handling.

In this testcase, the primary evaluation successfully produces 'true', and
then running one of the cleanups hits a double delete, making the whole
thing not a valid constant expression.  So we were returning 'true' wrapped
in a NOP_EXPR to indicate its non-constancy, but evaluating that again is a
perfectly acceptable constant expression, so we weren't getting the verbose
diagnostic we were looking for.

So if non_constant_p gets set other than for overflow, go back to the
original expression.

With this change, we should never hit the manifestly_const_eval test, and
the is-constant-evaluated1.C test passes without it.

gcc/cp/ChangeLog:

	PR c++/97388
	* constexpr.c (cxx_eval_outermost_constant_expr): Revert to
	original expression if evaluation sets non_constant_p.

gcc/testsuite/ChangeLog:

	PR c++/97388
	* g++.dg/cpp2a/constexpr-dtor8.C: New test.

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