New SetOperations and unittesting for all SetOperations

New set operations split out of D140908 as suggested, and I have added
unit testing for all set operations.

This adds a set_intersection, which returns the intersection instead of
updating the first set like set_intersect (using a different name
analogous to set_difference vs set_subtract).

Also adds a variant of set_subtract that updates two additional set
arguments to note which members of the subtrahend were removed from
the minuend and which were not.

Differential Revision: https://reviews.llvm.org/D144220