swift234

Use a bump pointer allocator in the callee set creation.

Shaves about 19% of the time from the construction of these sets. The
SmallVector size was chosen to minimize the number of dynamic
allocations we end up doing while building the stdlib. This should be a
reasonable size for most projects, too. It's a bit wasteful in space,
but the total amount of allocated space here is pretty small to begin
with.