swift234

Fix a couple of common crashers related to functions with default arguments. (rdar://problem/21799331 and rdar://problem/21643052, plus dupes.)

If a function declaration possessed default parameters, and was invoked with a single argument expression that was modeled as a type variable, the compiler would often crash during type application.  This was due to the fact that during simplification, we would bind the type variable to the full tuple type of the parameter list. Later on, during constraint application, we would then look to whatever expression created the type variable for information on its default arguments - even if no such thing was possible. (E.g., we would examine, say, an IfExpr expecting to find information on its default arguments.) In these cases, we should have instead been binding the argument type variable to the first element of the parameter tuple.

Swift SVN r30486