swift234

AST: Shun empty GenericSignatures, and remove the ASTContext argument from GenericSignature factories.

GenericSignatures with no params or requirements are a bug, so verify that they don't happen by making GenericSignature::get return null and GenericFunctionType assert that it has a nonnull signature. Hack Sema not to try to produce nongeneric GenericFunctionTypes when a function in a local type in a generic function context is type-checked; there's a deeper modeling issue that needs to be fixed here, but that's beyond the scope of 1.0. Now that GenericSignature always has at least one subtype, its factories no longer need an independent ASTContext argument.

Swift SVN r13837