c++: alias template and parameter packs (PR91966).

In this testcase, when we do a pack expansion of count_better_mins<nums>,
nums appears both in the definition of count_better_mins and as its template
argument.  The intent is that we get a expansion over pairs of elements of
the pack, i.e. less<2,2>, less<2,7>, less<7,2>, ....  But if we substitute
into the definition of count_better_mins when parsing the template, we end
up with sum<less<nums,nums>...>, which never gives us less<2,7>.  We could
deal with this by somehow marking up the use of 'nums' as an argument for
'num', but it's simpler to mark the alias as complex, so we need to
instantiate it later with all its arguments rather than replace it early
with its expansion.

gcc/cp/ChangeLog
2020-04-03  Jason Merrill  <jason@redhat.com>

	PR c++/91966
	* pt.c (complex_pack_expansion_r): New.
	(complex_alias_template_p): Use it.

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