[stdlib] Changing the deprecation message for StringInterpolationConvertible

Since the ExpressibleByStringInterpolation protocol is also deprecated
now, it makes little sense to suggest using it instead of
StringInterpolationConvertible. Instead, the message now recommends
considering an init(_:String).
See
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160808/026171.html
for some more details.

Author: Max Moiseev

stdlib/public/core/CompilerProtocols.swift

@@ -737,7 +737,7 @@ public typealias ArrayLiteralConvertible
 @available(*, deprecated, renamed: "ExpressibleByDictionaryLiteral")
 public typealias DictionaryLiteralConvertible
   = ExpressibleByDictionaryLiteral
-@available(*, deprecated, renamed: "ExpressibleByStringInterpolation")
+@available(*, deprecated, message: "it will be replaced or redesigned in Swift 4.0.  Instead of conforming to 'StringInterpolationConvertible', consider adding an 'init(_:String)'")
 public typealias StringInterpolationConvertible
   = ExpressibleByStringInterpolation
 @available(*, deprecated, renamed: "_ExpressibleByColorLiteral")