c++: constexpr init of union sub-aggr w/ base [PR105491]

c++: constexpr init of union sub-aggr w/ base [PR105491]

Here ever since r10-7313-gb599bf9d6d1e18, reduced_constant_expression_p
in C++11/14 is rejecting the marked sub-aggregate initializer (of type S)

  W w = {.D.2445={.s={.D.2387={.m=0}, .b=0}}};
                     ^
ultimately because said initializer has CONSTRUCTOR_NO_CLEARING set,
hence the function must verify that all fields of S are initialized.
And before C++17 it doesn't expect to see base class fields (since
next_initializable_field skips over them), so the presence thereof
causes r_c_e_p to return false.

The reason r10-7313-gb599bf9d6d1e18 causes this is because in that
commit we began using CONSTRUCTOR_NO_CLEARING to precisely track whether
we're in middle of activating a union member.  This ends up affecting
clear_no_implicit_zero, which recurses into sub-aggregate initializers
only if the outer initializer has CONSTRUCTOR_NO_CLEARING set.  After
that commit, the outer union initializer above no longer has the flag
set at this point and so clear_no_implicit_zero no longer recurses into
the marked inner initializer.

But arguably r_c_e_p should be able to accept the marked initializer
regardless of whether CONSTRUCTOR_NO_CLEARING is set.  The primary bug
therefore seems to be that r_c_e_p relies on next_initializable_field
which skips over base class fields in C++11/14.  To fix this, this patch
introduces a new helper function next_subobject_field which is like
next_initializable_field except that it never skips base class fields,
and makes r_c_e_p use it.  This patch then renames next_initializable_field
to next_aggregate_field (and makes it skip over vptr fields again).

NB: This minimal backport of r13-211-g0c7bce0ac184c0 for 12.2 just adds
next_subobject_field and makes reduced_constant_expression_p use it.

	PR c++/105491

gcc/cp/ChangeLog:

	* constexpr.cc (reduced_constant_expression_p): Use
	next_subobject_field instead.
	* cp-tree.h (next_subobject_field): Declare.
	* decl.cc (next_subobject_field): Define.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-union7.C: New test.
	* g++.dg/cpp0x/constexpr-union7a.C: New test.
	* g++.dg/cpp2a/constinit17.C: New test.

(cherry picked from commit 0c7bce0ac184c057bacad9c8e615ce82923835fd)