[c++-interop] Improve performance of creating a C++ std::string from a contiguous UTF-8 Swift.String

Author: kongkaikai

stdlib/public/Cxx/std/String.swift

@@ -20,11 +20,17 @@ extension std.string {
   /// - Complexity: O(*n*), where *n* is the number of UTF-8 code units in the
   ///   Swift string.
   public init(_ string: String) {
-    self.init()
-    let utf8 = string.utf8
-    self.reserve(utf8.count)
-    for char in utf8 {
-      self.push_back(value_type(bitPattern: char))
+    if string.isContiguousUTF8 {
+      self = string.withCString(encodedAs: UTF8.self) { buffer in
+        Self(buffer, string.utf8.count, .init())
+      }
+    } else {
+      self.init()
+      let utf8 = string.utf8
+      self.reserve(utf8.count)
+      for char in utf8 {
+        self.push_back(value_type(bitPattern: char))
+      }
     }
   }