swift234

Add parsing and SILGen support for @final methods and properties.

This is missing almost all semantic analysis and is missing various
optimization opportunities (e.g. final methods that are not overrides
don't need vtable entries), but this is enough to devirtualize class
stuff, which is important for our performance efforts.  I'll add this
to release notes when it is more fully fleshed out.



Swift SVN r15885