c++: Fix up constexpr catching of pointer-to-members [PR126918]

The following patch fixes various problems with constexpr EH related to
pointer-to-member types.
http://eel.is/c++draft/except.handle#3.3
and
http://eel.is/c++draft/except.handle#3.4
have some cases where the exception object type and handler type can be
different.
If handler type is a pointer type, this is implemented by __cxa_begin_catch
returning the pointer by value rather than reference (pointer to the value
actually), so the cast is done during returning the pointer.
Unfortunately, for pointer-to-member types (both data and function) that is
not the case, __cxa_begin_catch in that case returns pointer to the
pointer-to-member type.
At runtime libsupc++/pbase_type_info.cc (__do_catch) deals with the
[except.handle]/(3.4) cases
      else if (typeid (*this) == typeid(__pointer_to_member_type_info))
        {
          if (__pointee->__is_function_p ())
            {
              using pmf_type = void (__pbase_type_info::*)();
              static const pmf_type pmf = nullptr;
              *thr_obj = const_cast<pmf_type*>(&pmf);
              return true;
            }
          else
            {
              using pm_type = int __pbase_type_info::*;
              static const pm_type pm = nullptr;
              *thr_obj = const_cast<pm_type*>(&pm);
              return true;
            }
        }
and [except.handle]/(3.3) cases are done presumably by strict aliasing
violation not visible to the compiler (runtime library returns address of
the exception type and compiler emitted code reads it using different
effective type).  E.g. for the foo case in the first testcase, the IL looks
like
   <<< Unknown tree: handler

      {
        <<< Unknown tree: offset_type >>> p = *(<<< Unknown tree: offset_type >>> &) D.2709;

        try
          {
                        register <<< Unknown tree: offset_type >>> * D.2709;
            <<cleanup_point <<< Unknown tree: expr_stmt
              (void) (D.2709 = (<<< Unknown tree: offset_type >>> *) __cxa_begin_catch (__builtin_eh_pointer (0))) >>>>>;
                        <<< Unknown tree: offset_type >>> p = *(<<< Unknown tree: offset_type >>> &) D.2709;
            return <retval> = p;
          }
        finally
          {
            __cxa_end_catch ();
          }
      } >>>
so my attempt to use a TARGET_EXPR for the temporary didn't work,
there is a CLEANUP_POINT_EXPR wrapping the D.2709 = __cxa_begin_catch (...)
assignment created from cp_finish_decl that would be quite hard to avoid (we
already do that through ugly hacks for structured binding CWG2867 support,
but it has consequences for e.g. coroutines etc.).
The following patch instead creates special temporaries (as if heap
allocated but more efficiently) that live just from the __cxa_begin_catch
(or __cxa_get_exception_ptr) time to the corresponding __cxa_end_catch.
They are stored in the caught_exception vector because attaching them
as DECL_CHAIN of the exception object looks unsafe to me, the current
exception could be queried and thrown again before the catch parameter
goes out of scope.

Also, I had to tweak handler_match_for_exception_type, because it only
handled pointer-to-data-member and not all pointer-to-member types
that [except.handle]/3 requires.

2026-08-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/126918
	* constexpr.cc (class constexpr_global_ctx): Extend description of
	caught_exceptions vector.
	(cxx_eval_cxa_builtin_fn): When catching a pointer-to-member and
	the current exception is nullptr or pointer-to-member with different
	type, create a temporary, initialize it from the exception value and
	return address of it.  Make sure to free these temporaries at
	__cxa_end_catch time.
	* call.cc (handler_match_for_exception_type): Allow NULLPTR_TYPE
	exception type or different pointer-to-member type even for
	TYPE_PTRMEMFUNC_P types, not just TYPE_PTRDATAMEM_P types.

	* g++.dg/cpp26/constexpr-eh24.C: New test.
	* g++.dg/cpp26/constexpr-eh25.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 8aa30b41ec381846e4c08b78d437fd896d30f5da)