We can't just recursively process types without caching. Instead mark a type as
safe before we recurse and reset this assumption if we proved otherwise on the
recursive traversal.
rdar://20132313
Swift SVN r26048
This adds an analysis to the compiler that identifies types that are may store
to memory on destruction.
It adds a compiler known protocol _DestructorSafeContainer that allows the
standard library to identify containers whose destructor's memory effects
depends strictly on the type parameters of the container.
Array<T> : _DestructorSafeContainer {} may not store to memory during
destruction if the bound T is a type that does not store to memory on
destruction.
This is needed to deduce that for example Array<Array<Int>> is does not store to
memory on destruction (e.g during a call to release).
rdar://18940376
Swift SVN r23242
