Following Jordan's advice, replace "join" with "union" and "meet" with
"intersect" in VersionRange to make it clear which direction these operations
go in the lattice of ranges.
Swift SVN r31640
Add a join (least upper bound) operation to the availability version range
lattice. A later commit will use this to reason about the disjunctive
else-branch flow for conditions with multiple conjunctive statement conditions.
This is part of rdar://problem/22307360.
Swift SVN r31610
