c++: Fix ICE on constexpr placement new [PR115754]

C++26 is making in P2747R2 paper placement new constexpr.
While working on a patch for that, I've noticed we ICE starting with
GCC 14 on the following testcase.
The problem is that e.g. for the void * to sometype * casts checks,
we really assume the casts have their operand constant evaluated
as prvalue, but on the testcase the cast itself is evaluated with
vc_discard and that means op can end up e.g. a VAR_DECL which the
later code doesn't like and asserts on.
If the result type is void, we don't really need the cast operand
for anything, so can use vc_discard for the recursive call,
VIEW_CONVERT_EXPR can appear on the lhs, so we need to honor the
lval but otherwise the patch uses vc_prvalue.
I'd like to get this patch in before the rest of P2747R2 implementation,
so that it can be backported to 14.2 later on.

2024-07-02  Jakub Jelinek  <jakub@redhat.com>
	    Jason Merrill  <jason@redhat.com>

	PR c++/115754
	* constexpr.cc (cxx_eval_constant_expression) <case CONVERT_EXPR>:
	For conversions to void, pass vc_discard to the recursive call
	and otherwise for tcode other than VIEW_CONVERT_EXPR pass vc_prvalue.

	* g++.dg/cpp26/pr115754.C: New test.

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