Fix CLANG compilation: folly/TokenBucket.h

Summary:
In the next revision of clang, compilation generates multiple erroes of the form

   folly/TokenBucket.h:127:29: error: exception specification is not available until end of  class definition
         double nowInSeconds = defaultClockNow()) {

which are eliminated by reordering the method declaration for defaultClockNow()

Refer https://llvm.org/bugs/show_bug.cgi?id=30860

Reviewed By: philippu, yfeldblum

Differential Revision: D4107443

fbshipit-source-id: ce64f2ae7983e533c2fcb5cb043dbdd3da5c00f7

folly/TokenBucket.h

@@ -104,6 +104,15 @@ class ParameterizedDynamicTokenBucket {
     zeroTime_ = zeroTime;
   }
 
+  /**
+   * Returns the current time in seconds since Epoch.
+   */
+  static double defaultClockNow() noexcept(noexcept(ClockT::timeSinceEpoch())) {
+    return std::chrono::duration_cast<std::chrono::duration<double>>(
+               ClockT::timeSinceEpoch())
+        .count();
+  }
+
   /**
    * Attempts to consume some number of tokens. Tokens are first added to the
    * bucket based on the time elapsed since the last attempt to consume tokens.
@@ -191,15 +200,6 @@ class ParameterizedDynamicTokenBucket {
     return std::min((nowInSeconds - this->zeroTime_) * rate, burstSize);
   }
 
-  /**
-   * Returns the current time in seconds since Epoch.
-   */
-  static double defaultClockNow() noexcept(noexcept(ClockT::timeSinceEpoch())) {
-    return std::chrono::duration_cast<std::chrono::duration<double>>(
-               ClockT::timeSinceEpoch())
-        .count();
-  }
-
  private:
   template <typename TCallback>
   bool consumeImpl(
@@ -268,6 +268,13 @@ class ParameterizedTokenBucket {
   ParameterizedTokenBucket& operator=(
       const ParameterizedTokenBucket& other) noexcept = default;
 
+  /**
+   * Returns the current time in seconds since Epoch.
+   */
+  static double defaultClockNow() noexcept(noexcept(Impl::defaultClockNow())) {
+    return Impl::defaultClockNow();
+  }
+
   /**
    * Change rate and burst size.
    *
@@ -370,13 +377,6 @@ class ParameterizedTokenBucket {
     return burstSize_;
   }
 
-  /**
-   * Returns the current time in seconds since Epoch.
-   */
-  static double defaultClockNow() noexcept(noexcept(Impl::defaultClockNow())) {
-    return Impl::defaultClockNow();
-  }
-
  private:
   Impl tokenBucket_;
   double rate_;