c++: Fix up build_data_member_initialization [PR121445]

The following testcase ICEs, because the constexpr ctor in C++14
or later doesn't contain any member initializers and so the
massage_constexpr_body -> build_constexpr_constructor_member_initializers
-> build_data_member_initialization member initialization discovery
looks at the ctor body instead.  And while it has various
cases where it punts, including COMPONENT_REF with a VAR_DECL as first
operand on lhs of INIT_EXPR, here there is COMPONENT_REF with
several COMPONENT_REFs and VAR_DECL only inside the innermost.

The following patch makes sure we punt on those as well, instead of
blindly assuming it is anonymous union member initializer or asserting
it is a vtable store.

An alternative to this would be some flag on the INIT_EXPRs created
by perform_member_init and let build_data_member_initialization inspect
only INIT_EXPRs with that flag set.

2025-11-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/121445
	* constexpr.cc (build_data_member_initialization): Just return
	false if member is COMPONENT_REF of COMPONENT_REF with
	VAR_P get_base_address.

	* g++.dg/cpp1y/constexpr-121445.C: New test.

(cherry picked from commit 0ac5a60c37d9a40e8993b8207e904b713e73b5d3)