c++: Add some conversion sanity checking.

c++: Add some conversion sanity checking.

Another change I was working on revealed that for complex numbers we were
building a ck_identity with build_conv, leading to the wrong active member
in the union being set.  Rather than add another enumeration of the
appropriate conversion codes, I factored that out.

gcc/cp/ChangeLog:

	* call.c (has_next): Factor out from...
	(next_conversion): ...here.
	(strip_standard_conversion): And here.
	(is_subseq): And here.
	(build_conv): Check it.
	(standard_conversion): Don't call build_conv
	for ck_identity.