swift234

Clean up inference of default arguments from imported APIs (mostly).

My temporary hackery around inferring default arguments from imported
APIs was too horrible. Make it slightly more sane by:

1) Actually marking these as default arguments in the type system,
rather than doing everything outside of the type system. This is a
step closer to what we would really do, if we go in this
direction. Put it behind the new -frontend flag
-enable-infer-default-arguments.

2) Only inferring a default argument from option sets and from
explicitly "nullable" parameters, as stated in the (Objective-)C API
or API notes. This eliminates a pile of spurious, non-sensical "=
nil"'s in the resulting output.

Note that there is one ugly tweak to the overloading rules to prefer
declarations with fewer defaulted arguments. This is a bad
implementation of what is probably a reasonable rule (prefer to bind
fewer default arguments), which intentionally only kicks in when we're
dealing with imported APIs that have default arguments.

Swift SVN r32078