libstdc++: Implement proposed resolution of LWG4477 [PR122671]

This patch implements the proposed resolution of
https://cplusplus.github.io/LWG/issue4477
The PR complains that one can't successfully throw from constructor
in placement new in a constant expression and catch that exception
later on.  The problem is while P2747R2 made placement ::operator new
and ::operator new[] constexpr, when the ctor throws it invokes also
these weird placement ::operator delete and ::operator delete[]
which intentionally perform no action, and those weren't constexpr,
so constant expression evaluation failed.

2025-11-19  Jakub Jelinek  <jakub@redhat.com>

	PR libstdc++/122671
	* libsupc++/new (::operator delete, ::operator delete[]): Implement
	proposed LWG4477 resolution.  Use _GLIBCXX_PLACEMENT_CONSTEXPR for
	placement operator deletes.

	* g++.dg/cpp26/constexpr-eh17.C: New test.

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