c++: Fix ICE with { } as template argument [PR94592]

As an extension (there should be a CWG about this though), we support
braced-init-list as a template argument, but convert_nontype_argument
had trouble digesting them.  We ICEd because of the double coercion we
perform for template arguments: convert_nontype_argument called from
finish_template_type got a { }, and since a class type was involved and
we were in a template, convert_like created an IMPLICIT_CONV_EXPR.  Then
the second conversion of the same argument crashed in constexpr.c
because the IMPLICIT_CONV_EXPR had gotten wrapped in a TARGET_EXPR.
Another issue was that an IMPLICIT_CONV_EXPR leaked to constexpr.c when
building an aggregate init.

We should have instantiated the IMPLICIT_CONV_EXPR in the first call to
convert_nontype_argument, but we didn't, because the call to
is_nondependent_constant_expression returned false because it checks
!BRACE_ENCLOSED_INITIALIZER_P.  Then non_dep was false even though the
expression didn't contain anything dependent and we didn't instantiate
it in convert_nontype_argument.  To fix this, check
BRACE_ENCLOSED_INITIALIZER_P in cxx_eval_outermost_constant_expr rather
than in is_nondependent_*.

	PR c++/94592
	* constexpr.c (cxx_eval_outermost_constant_expr): Return when T is
	a BRACE_ENCLOSED_INITIALIZER_P.
	(is_nondependent_constant_expression): Don't check
	BRACE_ENCLOSED_INITIALIZER_P.
	(is_nondependent_static_init_expression): Likewise.

	* g++.dg/cpp2a/nontype-class34.C: New test.
	* g++.dg/cpp2a/nontype-class35.C: New test.

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