[cxx-interop] Conform std::u16string to CustomStringConvertible and CustomDebugStringConvertible

This conforms `std::u16string` to two protocols that `std::string` already conforms to.

rdar://106037638

Author: egorzhdan

stdlib/public/Cxx/std/String.swift

@@ -53,18 +53,32 @@ extension std.u16string: ExpressibleByStringLiteral {
   }
 }
 
+// MARK: Getting a Swift description of a C++ string
+
 extension std.string: CustomDebugStringConvertible {
   public var debugDescription: String {
     return "std.string(\(String(self)))"
   }
 }
 
+extension std.u16string: CustomDebugStringConvertible {
+  public var debugDescription: String {
+    return "std.u16string(\(String(self)))"
+  }
+}
+
 extension std.string: CustomStringConvertible {
   public var description: String {
     return String(self)
   }
 }
 
+extension std.u16string: CustomStringConvertible {
+  public var description: String {
+    return String(self)
+  }
+}
+
 // MARK: Initializing Swift String from a C++ string
 
 extension String {

test/Interop/Cxx/stdlib/overlay/std-string-overlay.swift

@@ -90,6 +90,21 @@ StdStringOverlayTestSuite.test("std::string as Swift.CustomDebugStringConvertibl
   expectEqual(cxx3.debugDescription, "std.string(���)")
 }
 
+StdStringOverlayTestSuite.test("std::u16string as Swift.CustomDebugStringConvertible") {
+  let cxx1 = std.u16string()
+  expectEqual(cxx1.debugDescription, "std.u16string()")
+
+  let cxx2 = std.u16string("something123")
+  expectEqual(cxx2.debugDescription, "std.u16string(something123)")
+
+  let scalars: [UInt16] = [97, 55296, 99]
+  var cxx3 = std.u16string()
+  for scalar in scalars {
+    cxx3.push_back(scalar)
+  }
+  expectEqual(cxx3.debugDescription, "std.u16string(a�c)")
+}
+
 StdStringOverlayTestSuite.test("std::string as Swift.Sequence") {
   let cxx1 = std.string()
   var iterated = false
@@ -124,4 +139,19 @@ StdStringOverlayTestSuite.test("std::string as CustomStringConvertible") {
   expectEqual(cxx3.description, "���")
 }
 
+StdStringOverlayTestSuite.test("std::u16string as Swift.CustomStringConvertible") {
+  let cxx1 = std.u16string()
+  expectEqual(cxx1.description, "")
+
+  let cxx2 = std.u16string("something123")
+  expectEqual(cxx2.description, "something123")
+
+  let scalars: [UInt16] = [97, 55296, 99]
+  var cxx3 = std.u16string()
+  for scalar in scalars {
+    cxx3.push_back(scalar)
+  }
+  expectEqual(cxx3.description, "a�c")
+}
+
 runAllTests()