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/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;