New ALTER OPERATOR command fails to update dependencies

I noticed $subject while chasing a different issue.

The way to fix this is to export pg_operator.c's makeOperatorDependencies
function so that AlterOperator can call it.  But since that declaration
uses HeapTuple, I doubt we want to put it into pg_operator.h (which'd
require further growth in that file's include list, and possibly break
client-side code that wants to #include it).  I'm inclined to split out
pg_operator.h's function declarations into a new header pg_operator_fn.h,
as we've already done in related cases.

While this isn't exactly a stop-ship class of problem, it still seems like
it'd be good to fix before 9.5.0.

Any objections?
        regards, tom lane