c++: Fix ICE with unexpanded parameter pack [PR99844]

In explicit17.C, we weren't detecting an unexpanded parameter pack in
explicit(bool), so we crashed on a TEMPLATE_PARM_INDEX in constexpr.

I noticed the same is true for noexcept(), but only since my patch to
implement delayed parsing of noexcept.  Previously, we would detect the
unexpanded pack in push_template_decl but now the noexcept expression
has not yet been parsed, so we need to do it a bit later.

gcc/cp/ChangeLog:

	PR c++/99844
	* decl.c (build_explicit_specifier): Call
	check_for_bare_parameter_packs.
	* except.c (build_noexcept_spec): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/99844
	* g++.dg/cpp2a/explicit16.C: Use c++20.
	* g++.dg/cpp0x/noexcept66.C: New test.
	* g++.dg/cpp2a/explicit17.C: New test.