swift234

Perform name lookup for where clauses based on the generic parameter's context.

We were ending up looking in the parent context, but it didn't matter
because the parser pre-resolved the names of generic parameters. We
shouldn't be relying on the parser to do that.

Note that this regresses four compiler crashes, because they end up
looking back into their own generic parameter lists in unhealthy
ways. I'm going to temporarily burn some karma because of what this
enables...

Swift SVN r26688