swift234

For a complicated set of reasons, protocol methods will be modeled
as taking the "self" receiver as @inout in the requirement funcdecls,
whether they are @mutating or not.  This will allow us to generate better
code, eliminate MaterializeExpr, and have immutable arguments someday.

As such, I think I'm done hacking on this function.  Clean it up and document
its behavior, NFC.


Swift SVN r12117