c++: Fix Bases(args...)... base initialization [PR88580]

c++: Fix Bases(args...)... base initialization [PR88580]

When substituting into the arguments of a base initializer pack
expansion, tsubst_initializer_list uses a dummy EXPR_PACK_EXPANSION
in order to expand an initializer such as Bases(args)... into
Bases#{0}(args#{0}) and so on.  But when an argument inside the base
initializer is itself a pack expansion, as in Bases(args...)..., the
argument is already an EXPR_PACK_EXPANSION so we don't need to wrap it.
It's also independent from the outer expansion of Bases, so we need to
"multiplicatively" append the expansion of args... onto the argument
list of each expanded base.

gcc/cp/ChangeLog:

	PR c++/88580
	* pt.c (tsubst_initializer_list): Correctly handle the case
	where an argument inside a base initializer pack expansion is
	itself a pack expansion.

gcc/testsuite/ChangeLog:

	PR c++/88580
	* g++.dg/cpp0x/variadic182.C: New test.