swift234

Fix the major case of <rdar://problem/23641896> QoI: Strings in Swift cannot be indexed directly with integer offsets

This adds special tuned diagnostics to handle the case when someone attempts to index
a string with an integer value, which is pretty common to do for people new to Swift.o
That said, this only handles one special case, if there are other cases that are
common, I'd love to hear about them.  Subscripts are already handled with a different
approach, I'm not sure which is better, but I added a testcase for both of them.