swift234

Enhance diagnoseGeneralConversionFailure to understand that constraints have a
path associated with them, and to dig the expression the constraint refers to out
of the locator.  Also teach simplifyLocator how to simplify closureexpr results out.

This eliminates a class of completely bogus diagnostics where the types reported
don't make any sense, resolving a class of radars like 19821875, where we now 
produce excellent diagnostics.

That said, we still pick constraints to report that are unfortunate in some cases, 
such as the example in expr/closure/closures.swift.


Swift SVN r29757