c++: Avoid a suspicious -Wnoexcept warning [PR93805]

c++: Avoid a suspicious -Wnoexcept warning [PR93805]

In this PR we're emitting -Wnoexcept warnings about potentially-throwing NSDMIs
when computing the noexcept specification of a class's defaulted default
constructor.  Although these warnings are in some sense valid, this patch takes
the route of suppressing them, because:

  1. the warning message is confusing in its current form;
  2. warning for 'struct C { B b = B(); };' but not for 'struct C { B b; };'
     is inconsistent; and
  3. emitting a warning here arguably doesn't fall under the umbrella of
     -Wnoexcept, whose documentation says it warns only when a
     noexcept-expression evaluates to false, but there are no
     noexcept-expressions here.

gcc/cp/ChangeLog:

	PR c++/93805
	* except.c (maybe_noexcept_warning): Add TODO comment.
	* method.c (walk_field_subobs): Pass tf_none to expr_noexcept_p.

gcc/testsuite/ChangeLog:

	PR c++/93805
	* g++.dg/warn/Wnoexcept2.C: New test.