Subzero: Allow builds with assertions disabled.

1. Setting command-line make variable NOASSERT=1 adds -DNDEBUG and builds in a separate directory.  By default, we still get Release+Asserts.

2. Add "(void)foo;" as necessary when foo is only used in an assert(), to remove warnings.

3. Minimize inclusion of llvm/Support/Timer.h because it adds warnings.

4. Call validateLiveness() only when asserts are enabled, because it's relatively expensive.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/623493002

src/IceTimerTree.h

diff --git a/src/IceTimerTree.h b/src/IceTimerTree.h
index 3162587..114cc6f 100644
--- a/src/IceTimerTree.h
+++ b/src/IceTimerTree.h
@@ -50,10 +50,7 @@
 
 private:
   void update();
-  static double timestamp() {
-    // TODO: Implement in terms of std::chrono for C++11.
-    return llvm::TimeRecord::getCurrentTime(false).getWallTime();
-  }
+  static double timestamp();
   const double FirstTimestamp;
   double LastTimestamp;
   uint64_t StateChangeCount;