[test] Make string internal testing a little more robust; NFC

Add an isSmall query to Character so testing doesn't have to bake in
internal format. Clarify the purpose of the invalid UTF-16 backdoor
creation method.

stdlib/public/core/Character.swift

@@ -394,6 +394,12 @@ extension Character {
     guard case .large(let storage) = _representation else { return nil }
     return storage
   }
+
+  @usableFromInline // @testable
+  internal var _isSmall: Bool {
+    guard case .smallUTF16(_) = _representation else { return false }
+    return true
+  }
 }
 
 extension Character {

stdlib/public/core/StringCreate.swift

@@ -172,4 +172,13 @@ extension String {
     return String(_largeStorage: storage)
   }
 
+  // For testing purposes only, allow ourselves to have invalid contents
+  @usableFromInline // @testable
+  static internal
+  func _fromInvalidUTF16(_ cus: UnsafeBufferPointer<UInt16>) -> String {
+    let storage = _SwiftStringStorage<UTF16.CodeUnit>.create(
+      capacity: cus.count, count: cus.count)
+    _ = storage._initialize(fromCodeUnits: cus, encoding: UTF16.self)
+    return String(_StringGuts(_large: storage))
+  }
 }

stdlib/public/core/StringGuts.swift

@@ -1255,19 +1255,6 @@ extension _StringGuts {
   }
 }
 
-extension _StringGuts {
-  // For testing purposes only. Might be both inefficient and too low-level.
-  // There should be an eventual API on String to accomplish something similar.
-  @usableFromInline // @_testable
-  static internal
-  func _createStringFromUTF16(_ cus: UnsafeBufferPointer<UInt16>) -> String {
-    let storage = _SwiftStringStorage<UTF16.CodeUnit>.create(
-      capacity: cus.count, count: cus.count)
-    _ = storage._initialize(fromCodeUnits: cus, encoding: UTF16.self)
-    return String(_StringGuts(_large: storage))
-  }
-}
-
 extension _SwiftStringStorage {
   /// Initialize a piece of freshly allocated storage instance from a sequence
   /// of code units, which is assumed to contain exactly as many code units as

test/stdlib/Character.swift

@@ -231,12 +231,7 @@ func checkRoundTripThroughCharacter(_ s: String) {
 }
 
 func isSmallRepresentation(_ s: String) -> Bool {
-  switch Character(s)._representation {
-    case .smallUTF16:
-      return true
-    default:
-      return false
-  }
+  return Character(s)._isSmall
 }
 
 func checkUnicodeScalars(_ s: String) {

validation-test/stdlib/StringViews.swift

@@ -37,8 +37,8 @@ let replacementCharacter = Character(replacementScalar)
 // UTF16, grapheme clusters composed of multiple Unicode scalars, and
 // invalid UTF16 that should be replaced with replacement characters.
 let winterUTF16 = Array("🏂☃❅❆❄︎⛄️❄️".utf16) + [0xD83C, 0x0020, 0xDF67, 0xD83C]
-var winter = winterUTF16.withUnsafeBufferPointer { bufPtr in 
-  _StringGuts._createStringFromUTF16(bufPtr)
+var winter = winterUTF16.withUnsafeBufferPointer {
+  String._fromInvalidUTF16($0)
 }
 
 let winterInvalidUTF8: [UTF8.CodeUnit] = replacementUTF8 + ([0x20] as [UTF8.CodeUnit]) + replacementUTF8 + replacementUTF8