swift234

SIL.rst: Describe instructions for existentials.
We'll need some additional instructions to describe initialization of existentials and existential method calls: `alloc_existential` to initialize an existential's witness and substitution table(s) and provide access to its storage, and `existential_method_ref`/`project_existential` to obtain a method handle from an existential and the opaque reference to hand down to the method.

Swift SVN r3348