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/runtime/base/allocator.h b/runtime/base/allocator.h
index e48eca9..8d1c982 100644
--- a/runtime/base/allocator.h
+++ b/runtime/base/allocator.h
@@ -116,7 +116,8 @@
 
   // Used internally by STL data structures.
   template <class U>
-  TrackingAllocatorImpl(const TrackingAllocatorImpl<U, kTag>& alloc ATTRIBUTE_UNUSED) noexcept {}
+  TrackingAllocatorImpl( // NOLINT, implicit
+      const TrackingAllocatorImpl<U, kTag>& alloc ATTRIBUTE_UNUSED) noexcept {}
 
   // Used internally by STL data structures.
   TrackingAllocatorImpl() noexcept {
diff --git a/runtime/base/arena_allocator.h b/runtime/base/arena_allocator.h
index 3fad96b..31dbb36 100644
--- a/runtime/base/arena_allocator.h
+++ b/runtime/base/arena_allocator.h
@@ -258,9 +258,9 @@
 
 class ArenaPool {
  public:
-  ArenaPool(bool use_malloc = true,
-            bool low_4gb = false,
-            const char* name = "LinearAlloc");
+  explicit ArenaPool(bool use_malloc = true,
+                     bool low_4gb = false,
+                     const char* name = "LinearAlloc");
   ~ArenaPool();
   Arena* AllocArena(size_t size) REQUIRES(!lock_);
   void FreeArenaChain(Arena* first) REQUIRES(!lock_);
diff --git a/runtime/base/arena_containers.h b/runtime/base/arena_containers.h
index 68cacd5..2c8aa28 100644
--- a/runtime/base/arena_containers.h
+++ b/runtime/base/arena_containers.h
@@ -132,7 +132,7 @@
         arena_allocator_(arena_allocator) {
   }
   template <typename U>
-  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)
+  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : ArenaAllocatorAdapterKind(other),
         arena_allocator_(other.arena_allocator_) {
   }
@@ -168,7 +168,7 @@
         arena_allocator_(arena_allocator) {
   }
   template <typename U>
-  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)
+  ArenaAllocatorAdapter(const ArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : ArenaAllocatorAdapterKind(other),
         arena_allocator_(other.arena_allocator_) {
   }
diff --git a/runtime/base/scoped_arena_containers.h b/runtime/base/scoped_arena_containers.h
index bd19d00..7964705 100644
--- a/runtime/base/scoped_arena_containers.h
+++ b/runtime/base/scoped_arena_containers.h
@@ -87,7 +87,7 @@
         arena_stack_(arena_allocator->arena_stack_) {
   }
   template <typename U>
-  ScopedArenaAllocatorAdapter(const ScopedArenaAllocatorAdapter<U>& other)
+  ScopedArenaAllocatorAdapter(const ScopedArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : DebugStackReference(other),
         DebugStackIndirectTopRef(other),
         ArenaAllocatorAdapterKind(other),
@@ -130,7 +130,7 @@
         arena_stack_(arena_allocator->arena_stack_) {
   }
   template <typename U>
-  ScopedArenaAllocatorAdapter(const ScopedArenaAllocatorAdapter<U>& other)
+  ScopedArenaAllocatorAdapter(const ScopedArenaAllocatorAdapter<U>& other)  // NOLINT, implicit
       : DebugStackReference(other),
         DebugStackIndirectTopRef(other),
         ArenaAllocatorAdapterKind(other),
diff --git a/runtime/base/transform_array_ref.h b/runtime/base/transform_array_ref.h
index a4e0bc2..b432f86 100644
--- a/runtime/base/transform_array_ref.h
+++ b/runtime/base/transform_array_ref.h
@@ -72,7 +72,7 @@
 
   template <typename OtherBT,
             typename = typename std::enable_if<std::is_same<BaseType, const OtherBT>::value>::type>
-  TransformArrayRef(const TransformArrayRef<OtherBT, Function>& other)
+  TransformArrayRef(const TransformArrayRef<OtherBT, Function>& other)  // NOLINT, implicit
       : TransformArrayRef(other.base(), other.GetFunction()) { }
 
   // Assignment operators.
diff --git a/runtime/base/transform_iterator.h b/runtime/base/transform_iterator.h
index 9c8f822..f1a8a52 100644
--- a/runtime/base/transform_iterator.h
+++ b/runtime/base/transform_iterator.h
@@ -62,7 +62,7 @@
       : data_(base, fn) { }
 
   template <typename OtherBI>
-  TransformIterator(const TransformIterator<OtherBI, Function>& other)
+  TransformIterator(const TransformIterator<OtherBI, Function>& other)  // NOLINT, implicit
       : data_(other.base(), other.GetFunction()) {
   }