Frontend: Enable @_spiOnly by default in Swift 6
Enabling `@_spiOnly` also enables stronger type-checking of SPI decls.
As this could be source breaking, it has always been opt-in. Turn it on
by default in Swift 6 mode where the stronger type-checking will also
become expected.
At the same time, disable the old alternative to `@_spiOnly` which was
designed to be compatible with old compilers. Any user of that feature
should move to `@_spiOnly` or `package import` instead.
