swift234

Further improve type checker diagnostics through a variety of means.

Start capitalizing on some of the new diagnostic machinery in a few different ways:
- When mining constraints for type information, utilize constraints "favored" by the overload resolution process.
- When printing type variables, if the variable was created by opening a literal expression, utilize the literal
  default type or conformance if possible.
- Utilize syntactic information when crafting diagnostics:
	- If the constraint miner can produce a better diagnostic than the recorded failure, diagnose via constraints.
	- Factor in the expression kind when choosing which types to include in a diagnostic message.
- Start customizing diagnostics based on the amount of type data available.

What does all this mean?
- Fewer type variables leaking into diagnostic messages.
- Far better diagnostics for overload resolution failures. Specifically, we now print proper argument type data
  for failed function calls.
- No more "'Foo' is not convertible to 'Foo'" error messages
- A greater emphasis on type data means less dependence on the ordering of failed constraints. This means fewer
  inscrutable diagnostics complaining about 'UInt8' when all the constituent expressions are of type Float.

So we still have a ways to go, but these changes should greatly improve the number of head-scratchers served up
by the type checker.

These changes address the following radars:
rdar://problem/17618403
rdar://problem/17559042
rdar://problem/17007456
rdar://problem/17559042
rdar://problem/17590992
rdar://problem/17646988
rdar://problem/16979859
rdar://problem/16922560
rdar://problem/17144902
rdar://problem/16616948
rdar://problem/16756363
rdar://problem/16338509

Swift SVN r20927