[stdlib] Deprecate ExpressibleByStringInterpolation

We know its API is
inadequate (https://bugs.swift.org/browse/SR-1260?jql=text%20~%20%22StringInterpolationConvertible%22)
and don't want to be constrained to supporting it in future versions.

Author: Dave Abrahams

stdlib/public/core/CompilerProtocols.swift

@@ -613,6 +613,7 @@ public protocol ExpressibleByDictionaryLiteral {
 ///           String(stringInterpolationSegment: " cookies: $"),
 ///           String(stringInterpolationSegment: price * number),
 ///           String(stringInterpolationSegment: "."))
+@available(*, deprecated, message: "it will be replaced or redesigned in Swift 4.0.  Instead of conforming to 'ExpressibleByStringInterpolation', consider adding an 'init(_:String)'")
 public protocol ExpressibleByStringInterpolation {
   /// Creates an instance by concatenating the given values.
   ///

validation-test/stdlib/BoolDiagnostics_Dataflow.swift

@@ -55,3 +55,5 @@ func test_constantFoldOr4() -> Int {
   }
 } // expected-error {{missing return in a function expected to return 'Int'}}
 
+// expected-warning@+1 {{'ExpressibleByStringInterpolation' is deprecated: it will be replaced or redesigned in Swift 4.0.  Instead of conforming to 'ExpressibleByStringInterpolation', consider adding an 'init(_:String)'}}
+typealias X = ExpressibleByStringInterpolation