swift234

[docs] LibraryEvolution: @always_emit_into_client

Describe a new attribute, @always_emit_into_client, which covers the
behavior of helpers for inlineable functions: any client that makes use
of the body of an inlineable function should use that implementation,
including any helper functions, local functions, or closures referenced
therein. This is also a tool a library developer can use.

This is a nice change because it formalizes several things that were
previously special-cased; however, there are still some open questions.

It's not totally clear whether we want separate @inlineable and
@always_emit_into_client annotations; we could attempt to get away with
just the latter. However, as noted in the diff, there are some entities
that can't really be completely eliminated from the ABI, so marking them
@always_emit_into_client would be disingenuous.