We have for a long time talked about creating a scope like data structure for
use in the SILOptimizer. The discussion was whether or not to reuse the
infrastructure in SILGen that does this already. There were concerns about doing
so since the code in the SILOptimizer and SILGen can work differently.
With that in mind, I added a small AssertingScope class and built on top of that
a composition SIL level class called SILOptScope that one can use to add various
cleanups. One is able to both destructively pop at end of scope and pop along
early exits.
At an implementation level, I kept it simple and:
1. Represented a scope as a stack of Optional<Cleanup> which are just a wrapper
around a std::function. The Optional is so that we can invalidate a cleanup.
2. Based all of these scopes around the idea that the user of the scope must
invalidate the scope by hand. If not, the scope object will assert at the end
of its RAII scope.
3. Rather than creating a whole class hierarchy, I just used std::function
closures to keep things simple.
