Handle calling conventions and cleanups in all the places (hopefully).
- when ExistentialSpecializer copies the specialized concrete arg into the
original existential value
- when ExistentialSpecializer generates a think
- when SILCombine substitutes concrete values in place of the opened
existential.
One particularly nasty problem is the existential boxes need to be
destroyed. It is not ok to simply destroy their value. The "leaks"
tool does not catch this problem.
Ownership SIL will make this all much more robust.
Fixes <rdar://problem/50595630> Multiple leaks detected - Swift Perf
Andrew Trick
Raj Barik