swift234

Teach subscript candidate matching about the fact that self can mismatch with its available
value, and use that to rank a problem as very specific.  This required indicating a difference
between singular argument mismatch vs self mismatch and single-argument mismatch (which is very
specific) as being different from the argument list in general mismatching (which matters to
differentiate argument lists that contain a single argument).  

These extra mechanics combine to fix <rdar://problem/21362748> [WWDC Lab] QoI: cannot subscript a value of type '[Int]?' with an index of type 'Int'



Swift SVN r30305