Add a test for redundant protocol conformance.

Added a test that builds two dynamic libraries and sets up a redundant protocol
conformance (we use dynamic libraries because the compiler will normally reject
redundant conformances), then checks that when we rely on it, we get a warning
message.

Author: al45tair

test/multifile/Inputs/protocol-conformance-redundant-def.swift

@@ -0,0 +1,7 @@
+public protocol Hello {
+  func hello()
+}
+
+open class Super {
+  public init() {}
+}

test/multifile/Inputs/protocol-conformance-redundant-ext.swift

@@ -0,0 +1,7 @@
+import Def
+
+extension Super : Hello {
+  public func hello() {
+    print("Hello")
+  }
+}

test/multifile/protocol-conformance-redundant.swift

@@ -0,0 +1,39 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift-dylib(%t/%target-library-name(Def)) -module-name Def -emit-module -emit-module-path %t/Def.swiftmodule %S/Inputs/protocol-conformance-redundant-def.swift
+// RUN: %target-build-swift-dylib(%t/%target-library-name(Ext)) -module-name Ext -emit-module -emit-module-path %t/Ext.swiftmodule -I%t -L%t -lDef %S/Inputs/protocol-conformance-redundant-ext.swift
+// RUN: %target-build-swift -I%t -L%t -lDef -o %t/main %target-rpath(%t) %s
+// RUN: %target-codesign %t/main %t/%target-library-name(Def) %t/%target-library-name(Ext)
+// RUN: %target-run %t/main %t/%target-library-name(Ext) 2>&1 | %FileCheck %s
+
+// REQUIRES: executable_test
+// XFAIL: windows
+
+// CHECK: Warning: 'Sub' conforms to protocol 'Hello', but it also inherits conformance from 'Super'.  Relying on a particular conformance is undefined behaviour.
+// CHECK: Hello
+
+import StdlibUnittest
+
+#if canImport(Darwin)
+import Darwin
+#elseif canImport(Glibc)
+import Glibc
+#else
+#error("Unsupported platform")
+#endif
+
+import Def
+
+let dylibPath = CommandLine.arguments.last!
+let openRes = dlopen(dylibPath, RTLD_NOW|RTLD_LOCAL)
+assert(openRes != nil, "unable to open extension dylib")
+
+class Sub : Super, Hello {
+  func hello() {
+    print("Hello")
+  }
+}
+
+let s = Sub() as AnyObject as! Hello
+
+s.hello()
+