c++: Fix non-constant TARGET_EXPR constexpr handing [PR93998]

We ICE on the following testcase since I've added the SAVE_EXPR-like
constexpr handling where the TARGET_EXPR initializer (and cleanup) is
evaluated only once (because it might have side-effects like new or delete
expressions in it).
The problem is if the TARGET_EXPR (but I guess in theory SAVE_EXPR too)
initializer is *non_constant_p.  We still remember the result, but already
not that it is *non_constant_p.  Normally that wouldn't be a big problem,
if something is *non_constant_p, we only or into it and so the whole
expression will be non-constant too.  Except in the builtins handling,
we try to evaluate the arguments with non_constant_p pointing into a dummy1
bool which we ignore.  This is because some builtins might fold into a
constant even if they don't have a constexpr argument.  Unfortunately if
we evaluate the TARGET_EXPR first in the argument of such a builtin and then
once again, we don't set *non_constant_p.

So, either we don't remember the TARGET_EXPR/SAVE_EXPR result if it wasn't
constant, like the following patch does, or we could remember it, but in
some way that would make it clear that it is non-constant (e.g. by
pushing into the global->values SAVE_EXPR, SAVE_EXPR entry and perhaps
for TARGET_EXPR don't remember it on TARGET_EXPR_SLOT, but the TARGET_EXPR
itself and similarly push TARGET_EXPR, TARGET_EXPR and if we see those
after the lookup, diagnose + set *non_constant_p.  Or we could perhaps
during the builtin argument evaluation push expressions into a different
save_expr vec and undo them afterwards.

2020-03-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/93998
	* constexpr.c (cxx_eval_constant_expression)
	<case TARGET_EXPR, case SAVE_EXPR>: Don't record anything if
	*non_constant_p is true.

	* g++.dg/ext/pr93998.C: New test.

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