now that we have a simpler structure for the overal CSDiag algorithm,…

… change

diagnoseGeneralFailure to be named diagnoseConstraintFailure and change how
it works:
Now it ranks unresolved constraints  in the system based on kind (e.g. whether
they are favored, member constraints ahead of conversion constraints, etc) and
then tries to emit a diagnostic for each failure kind one after another.

This means that if there are multiple failed conversion constraints, but one
is obviously satisfiable, that we continue on to diagnose the next one.  This 
clears up a swath of embarassing diagnostics and refixes:
<rdar://problem/19658691> QoI: Incorrect diagnostic for calling nonexistent members on literals



Swift SVN r31046