feat: add comparable conformance for C++ strings

Author: ishon19

stdlib/public/Cxx/std/String.swift

@@ -161,6 +161,26 @@ extension std.u32string: Equatable {
   }
 }
 
+// MARK: Comparable conformance for C++ strings
+
+extension std.string: Comparable  {
+  public static func <(lhs: std.string, rhs: std.string) -> Bool {
+    return lhs.compare(rhs) < 0
+  }
+}
+
+extension std.u16string: Comparable  {
+  public static func <(lhs: std.u16string, rhs: std.u16string) -> Bool {
+    return lhs.compare(rhs) < 0
+  }
+}
+
+extension std.u32string: Comparable  {
+  public static func <(lhs: std.u32string, rhs: std.u32string) -> Bool {
+    return lhs.compare(rhs) < 0
+  }
+}
+
 // MARK: Hashing C++ strings
 
 extension std.string: Hashable {