Configure a basic .clang-tidy.

Also, fix some issues that it found.

We will enable more rules in the future. In particular, some google-*
rules should already pass, but appear not to at the moment.

Once we have enabled enough of these rules, we intend to drop the
dependency on cpplint.

Change-Id: I107231cf57b5500a3f1b623510419665589531dc
diff --git a/src/cpu-features.h b/src/cpu-features.h
index 012c5e4..50ddc26 100644
--- a/src/cpu-features.h
+++ b/src/cpu-features.h
@@ -333,8 +333,10 @@
   CPUFeatures::Feature feature_;
 
   bool IsValid() const {
-    return ((cpu_features_ == NULL) && (feature_ == CPUFeatures::kNone)) ||
-           cpu_features_->Has(feature_);
+    if (cpu_features_ == NULL) {
+      return feature_ == CPUFeatures::kNone;
+    }
+    return cpu_features_->Has(feature_);
   }
 };