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/arm/assembler_arm.h b/compiler/utils/arm/assembler_arm.h
index 3084e6e..aefbf26 100644
--- a/compiler/utils/arm/assembler_arm.h
+++ b/compiler/utils/arm/assembler_arm.h
@@ -246,7 +246,7 @@
     NegPostIndex = (0|0|0) << 21   // negative post-indexed with writeback
   };
 
-  Address(Register rn, int32_t offset = 0, Mode am = Offset) : rn_(rn), rm_(R0),
+  explicit Address(Register rn, int32_t offset = 0, Mode am = Offset) : rn_(rn), rm_(R0),
       offset_(offset),
       am_(am), is_immed_offset_(true), shift_(LSL) {
   }
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.
diff --git a/compiler/utils/swap_space.h b/compiler/utils/swap_space.h
index bf06675..9600907 100644
--- a/compiler/utils/swap_space.h
+++ b/compiler/utils/swap_space.h
@@ -114,7 +114,8 @@
   explicit SwapAllocator(SwapSpace* swap_space) : swap_space_(swap_space) {}
 
   template <typename U>
-  SwapAllocator(const SwapAllocator<U>& other) : swap_space_(other.swap_space_) {}
+  SwapAllocator(const SwapAllocator<U>& other)  // NOLINT, implicit
+      : swap_space_(other.swap_space_) {}
 
   SwapAllocator(const SwapAllocator& other) = default;
   SwapAllocator& operator=(const SwapAllocator& other) = default;
@@ -149,7 +150,8 @@
   explicit SwapAllocator(SwapSpace* swap_space) : swap_space_(swap_space) {}
 
   template <typename U>
-  SwapAllocator(const SwapAllocator<U>& other) : swap_space_(other.swap_space_) {}
+  SwapAllocator(const SwapAllocator<U>& other)  // NOLINT, implicit
+      : swap_space_(other.swap_space_) {}
 
   SwapAllocator(const SwapAllocator& other) = default;
   SwapAllocator& operator=(const SwapAllocator& other) = default;