Issue Description:

If you have code like:

struct S {
   init(_: UInt8) {}
   init?(_: Int) {}
}

let _: S = S(9999) // error: integer literal '9999' overflows when stored into 'UInt8'

If the user doesn't fully understand how type inference is working, this might be confusing (they might think that the Int overload ought to be called). For example, this can lead to potentially surprising behavior around Unicode.Scalar as in SR-13407.

It would be helpful in such a situation to point out the overload being used. To characterize the situation properly, here's when I'd expect this contextual information to kick in:

1. The error is due to overflow.

2. There are "similar" methods (e.g. other overloads or other initializers) for which the overflow would not happen.

If 2. is too difficult to implement, maybe we can special-case the single-argument initializer situation to address the Unicode.Scalar case and handle more complex situations later based on bug reports.