Fix google-explicit-constructor warnings in art.

* Add explicit keyword to conversion constructors,
  or NOLINT for implicit converters.
Bug: 28341362
Test: build with WITH_TIDY=1

Change-Id: I1e1ee2661812944904fedadeff97b620506db47d
diff --git a/compiler/utils/intrusive_forward_list.h b/compiler/utils/intrusive_forward_list.h
index ec2c087..b5fc2f2 100644
--- a/compiler/utils/intrusive_forward_list.h
+++ b/compiler/utils/intrusive_forward_list.h
@@ -59,7 +59,7 @@
   // Conversion from iterator to const_iterator.
   template <typename OtherT,
             typename = typename std::enable_if<std::is_same<T, const OtherT>::value>::type>
-  IntrusiveForwardListIterator(const IntrusiveForwardListIterator<OtherT, HookTraits>& src)
+  IntrusiveForwardListIterator(const IntrusiveForwardListIterator<OtherT, HookTraits>& src)  // NOLINT, implicit
       : hook_(src.hook_) { }
 
   // Iteration.