c++/modules: Stream BASELINK_OPTYPE [PR122381]

This is used in template conversion operators to determine what type the
user requested.

	PR c++/122381

gcc/cp/ChangeLog:

	* module.cc (trees_out::core_vals): Write BASELINK_OPTYPE (aka
	TREE_CHAIN).
	(trees_in::core_vals): Read it.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/convop-2_a.H: New test.
	* g++.dg/modules/convop-2_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
(cherry picked from commit b466450c6d502557921f03527cc165fb3402aaa9)