c++: ICE with template code in constexpr [PR104284]

Since r9-6073 cxx_eval_store_expression preevaluates the value to
be stored, and that revealed a crash where a template code (here,
code=IMPLICIT_CONV_EXPR) leaks into cxx_eval*.

It happens because we're performing build_vec_init while processing
a template, which calls get_temp_regvar which creates an INIT_EXPR.
This INIT_EXPR's RHS contains an rvalue conversion so we create an
IMPLICIT_CONV_EXPR.  Its operand is not type-dependent and the whole
INIT_EXPR is not type-dependent.  So we call build_non_dependent_expr
which, with -fchecking=2, calls fold_non_dependent_expr.  At this
point the expression still has an IMPLICIT_CONV_EXPR, which ought to
be handled in instantiate_non_dependent_expr_internal.  However,
tsubst_copy_and_build doesn't handle INIT_EXPR; it will just call
tsubst_copy which does nothing when args is null.  So we fail to
replace the IMPLICIT_CONV_EXPR and ICE.

The problem is that we call build_vec_init in a template in the
first place.  We can avoid doing so by checking p_t_d before
calling build_aggr_init in check_initializer.

	PR c++/104284

gcc/cp/ChangeLog:

	* decl.cc (check_initializer): Don't call build_aggr_init in
	a template.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/constexpr-104284-1.C: New test.
	* g++.dg/cpp1y/constexpr-104284-2.C: New test.
	* g++.dg/cpp1y/constexpr-104284-3.C: New test.
	* g++.dg/cpp1y/constexpr-104284-4.C: New test.