swift234

AST: Start unifying UnqualifiedLookup and lookupVisibleDecl()

For now, just update NameLookup's FindLocalVal to use a
VisibleDeclConsumer just like lookupVisibleDecl().

A subsequent patch will continue removing duplicated code
now that this is place.

This fixes compiler crashers where we were not handling
declarations with duplicate names (which of course is an
error, diagnosed elsewhere).

Swift SVN r29913