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) {
   }