[test] Add test for deserialization of generic protocol witnesses (#17141)

This failed in Swift 4.1 because of a problem with Substitutions,
but that whole model's been ripped out and redone in Swift 4.2.

https://bugs.swift.org/browse/SR-7978

Author: jrose-apple

validation-test/Serialization/SR7978.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -emit-module-path %t/Lib.swiftmodule %s -module-name Lib
+// RUN: echo 'import Lib; _ = MyClass(with: "test", and: 341)' | %target-build-swift -I %t -typecheck -
+
+public protocol MyProtocol {
+  var string: String { get }
+  var int: Int { get }
+
+  init(with string: String, and int: Int)
+
+  func doSomething<T, U>(with this: T, for string: String, expecting: U.Type) throws -> U
+  func doSomething<T>(with this: T, for string: String) throws
+}
+
+public class MyClass: MyProtocol {
+  public var string: String
+  public var int: Int
+
+  public required init(with string: String, and int: Int) {
+    self.string = string
+    self.int = int
+  }
+
+  public func doSomething<T>(with this: T, for string: String) throws {}
+  public func doSomething<T, U>(with this: T, for string: String, expecting: U.Type) throws -> U {
+    fatalError()
+  }
+}