swift234

Add a 'Builtin.once' builtin.

This lowers to a call to a to-be-written swift_once runtime function and will be used for lazy global initialization. Having this be a builtin seemed appropriate to me given that:

- references to it will be implicitly emitted by SILGen for global initializers.
- there are restrictions on its correct use that are currently impossible to express in the language outside of the stdlib.

Swift SVN r10508