NecessaryBindings are used by both async functions and partial apply
forwarders. The latter are able to avoid bindings in some cases because
a new function is generated where the information that would otherwise
be available in the bindings can be made available. That is not the
case for async functions. A generic async function requires all of the
metadata and witness tables be passed along to it: unlike a partial
apply forwarder it isn't in any way specialized so this information
can't be recovered.
Previously, metadata bindings were always passed along to async
functions. However, destructuring that can be done for partial apply
forwarders was still being applied. This resulted in an inappropriate
and unexpected number of bindings in NecessaryBindings.
Here, that destructuring is avoided for metadata passed to async
functions.
Now, the full metadata required by async functions are passed along to
them as necessary.
rdar://problem/71816041
