[cxx][windows] fix the CxxStdlib build for recent MSVC versions after recent regression

The build regressed after cbaef38 as newer versions of MSVC that aren't used in the OSS CI yet fail to build CxxStdlib now

Author: hyp

stdlib/public/Cxx/std/String.swift

@@ -21,13 +21,27 @@ extension std.string {
   ///   Swift string.
   public init(_ string: String) {
     self = string.withCString(encodedAs: UTF8.self) { buffer in
+#if os(Windows)
+      // Use the 2 parameter constructor.
+      // The MSVC standard library has a enable_if template guard
+      // on the 3 parameter constructor, and thus it's not imported into Swift.
+      std.string(buffer, string.utf8.count)
+#else
       std.string(buffer, string.utf8.count, .init())
+#endif
     }
   }
 
   public init(_ string: UnsafePointer<CChar>?) {
     if let str = string {
+#if os(Windows)
+      // Use the 2 parameter constructor.
+      // The MSVC standard library has a enable_if template guard
+      // on the 3 parameter constructor, and thus it's not imported into Swift.
+      self.init(str, UTF8._nullCodeUnitOffset(in: str))
+#else
       self.init(str, UTF8._nullCodeUnitOffset(in: str), .init())
+#endif
     } else {
       self.init()
     }