PR c++/89761 - ICE with sizeof... in pack expansion.

In this testcase we get confused when looking at the sizeof... because the
argument pack for 'args' has been wrapped in an ARGUMENT_PACK_SELECT as part
of expanding the fold-expression.  We handle this situation a bit lower down
in tsubst_pack_expansion, but that doesn't help the call to
argument_pack_element_is_expansion_p, which happens earlier.

	* pt.c (argument_pack_element_is_expansion_p): Handle
	ARGUMENT_PACK_SELECT.

From-SVN: r269776