swift234

implement support for a new [stdlib] attribute that can be slapped on an import decl.

This causes the SourceLoader to recursively parse the imported module in standard 
library mode, giving it access to the Builtin module.

This is all a terrible hack and should be ripped out with great victory someday, but 
until we have binary modules that persist the build setting used to produce the 
module, this is the best we can do.



Swift SVN r5847