c++: stream PACK_EXPANSION_EXTRA_ARGS [PR106761]

c++: stream PACK_EXPANSION_EXTRA_ARGS [PR106761]

It looks like after the libstdc++ commit r13-2158-g02f6b405f0e9dc
some xtreme-header-* tests are failing with "conflicting global module
declaration" errors ultimately because we're neglecting to stream
PACK_EXPANSION_EXTRA_ARGS, which leads to wrong equivalences of
different partial instantiations of _TupleConstraints::__constructible.

	PR c++/106761

gcc/cp/ChangeLog:

	* module.cc (trees_out::type_node) <case TYPE_PACK_EXPANSION>:
	Stream PACK_EXPANSION_EXTRA_ARGS.
	(trees_in::tree_node) <case TYPE_PACK_EXPANSION>: Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr106761.h: New test.
	* g++.dg/modules/pr106761_a.H: New test.
	* g++.dg/modules/pr106761_b.C: New test.