swift234

[value-tracking] Add an API to value tracking for determining dead closures.

I am going to use this in closure specialization to remove dead closures. We
should refactor the SILCombine dead closure removal to use this code path as
well.

Additionally, we should change the valueMayBeCaptured infrastructure to take a
struct like we do in LLVM so we can ignore casts and other things like that
additionally by injecting the logic to do so via the struct.

But that is for another day.

Swift SVN r25007