swift234

Various changes:
 - Don't "aka" a Builtin.Int2123 type, it just makes a bad diagnostic worse.
 - Split out the predicate that CSDiag uses to determine what a conversion
   constraint is to a helper fn, and add subtype constraints to the mix.
 - Move eraseTypeData into CSDiag (its own client) as a static function.
 - Make eraseTypeData be a bit more careful about literals, in an attempt to
   improve diagnostics when literals get re-type-checked.  It turns out that
   this still isn't enough as shown by the regression on the 
   decl/func/default-values.swift testcase, and the 
   Constraints/dictionary_literal.swift testcase where one bad diagnostic turns
   into another different one, but I'll keep working on it.
 - Beef up diagnoseContextualConversionError and the caller to it to be more
   self contained and principled about the conversion constraints it digs out
   of the system.  This improves the diagnostics on a couple of cases.



Swift SVN r30642