swift234

Change diagnoseFailure() for unavoidable failures to stop doing anything with 
conversion failures, making a bunch of diagnostics more specific and useful.

UnavoidableFailures can be very helpful, but they can also be the first constraint
failure that the system happened to come across... which is not always the most
meaningful one.  CSDiag's expr processing machinery has a generally better way of
narrowing down which ones make the most sense.


Swift SVN r30647