swift234

Add a lot of calling-convention annotations to the standard library / runtime.

The general rule here is that something needs to be SWIFT_CC(swift)
if it's just declared in Swift code using _silgen_name, as opposed to
importing something via a header.

Of course, SWIFT_CC(swift) expands to nothing by default for now, and
I haven't made an effort yet to add the indirect-result / context
parameter ABI attributes.  This is just a best-effort first pass.

I also took the opportunity to shift a few files to just implement
their shims header and to demote a few things to be private stdlib
interfaces.