libstdc++: Implement C++26 P2927R3 - Inspecting exception_ptr

libstdc++: Implement C++26 P2927R3 - Inspecting exception_ptr

The following patch attempts to implement the C++26 P2927R3 - Inspecting exception_ptr
paper (but not including P3748R0, I plan to play with it incrementally and
it will really depend on the Constexpr exceptions patch).

The function template is implemented using an out of line private method of
exception_ptr, so that P3748R0 then can use if consteval and provide a
constant evaluation variant of it.

2025-06-26  Jakub Jelinek  <jakub@redhat.com>

	* include/bits/version.def (exception_ptr_cast): Add.
	* include/bits/version.h: Regenerate.
	* libsupc++/exception: Define __glibcxx_want_exception_ptr_cast before
	including bits/version.h.
	* libsupc++/exception_ptr.h (std::exception_ptr_cast): Define.
	(std::__exception_ptr::exception_ptr::_M_exception_ptr_cast): Declare.
	* libsupc++/eh_ptr.cc
	(std::__exception_ptr::exception_ptr::_M_exception_ptr_cast): Define.
	* src/c++23/std.cc.in (std::exception_ptr_cast): Export.
	* config/abi/pre/gnu.ver: Export
	_ZNKSt15__exception_ptr13exception_ptr21_M_exception_ptr_castERKSt9type_info
	at CXXABI_1.3.17.
	* testsuite/util/testsuite_abi.cc (check_version): Allow CXXABI_1.3.17.
	* testsuite/18_support/exception_ptr/exception_ptr_cast.cc: New test.