Adjust PasswordCollector.describe() to return a const ref

Summary: This is mainly used for logging, and we can elide an unnecessary copy in some cases.

Reviewed By: mingtaoy

Differential Revision: D20402430

fbshipit-source-id: 79b73792a69cf2817ffb88b07d9973139c3c67bb

folly/io/async/PasswordInFile.h

@@ -33,7 +33,7 @@ class PasswordInFile : public PasswordCollector {
     return password_.c_str();
   }
 
-  std::string describe() const override {
+  const std::string& describe() const override {
     return fileName_;
   }
 

folly/io/async/SSLContext.h

@@ -58,7 +58,7 @@ class PasswordCollector {
   /**
    * Return a description of this collector for logging purposes
    */
-  virtual std::string describe() const = 0;
+  virtual const std::string& describe() const = 0;
 };
 
 /**