c++: Refine check for CTAD placeholder [PR99586]

c++: Refine check for CTAD placeholder [PR99586]

In the below testcase, during finish_compound_literal for A<B{V}>{},
type_uses_auto finds and returns the CTAD placeholder for B{V}, which
tricks us into attempting CTAD on A<B{V}>{} and leads to bogus errors.

AFAICT 'type' will always be a bare 'auto' in the CTAD case so we don't
need to look deeply to find it; checking template_placeholder_p instead
should suffice here.

gcc/cp/ChangeLog:

	PR c++/99586
	* semantics.c (finish_compound_literal): Check
	template_placeholder_p instead of type_uses_auto.

gcc/testsuite/ChangeLog:

	PR c++/99586
	* g++.dg/cpp2a/nontype-class42.C: New test.