[PATCH] Improve error reporting for SFINAE

[Paul Fultz II]

> gives the user no idea of how we got from a call to f into a call to g.


For the default case, its better to show the first and last in the trace, as most users don't care about the intermediate steps.

> If we produced a stack of 'in instantiation of' notes, this would be fine


Yes, the full back trace can be added in the future(perhaps as a compiler flag). This is the first step towards that direction. There needs to be some refactoring to `DeductionFailureInfo` so it stores the entire diagnostics(rather than a single diagnostic) to make a full back trace possible.

> note that's exactly what my patch in comment#1/comment#2 of that bug does


That seemed to be more of a hack then something to be used in production.


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http://reviews.llvm.org/D8309

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