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/optimizing/instruction_simplifier.h b/compiler/optimizing/instruction_simplifier.h
index 7905104..7fe1067 100644
--- a/compiler/optimizing/instruction_simplifier.h
+++ b/compiler/optimizing/instruction_simplifier.h
@@ -35,9 +35,9 @@
  */
 class InstructionSimplifier : public HOptimization {
  public:
-  InstructionSimplifier(HGraph* graph,
-                        OptimizingCompilerStats* stats = nullptr,
-                        const char* name = kInstructionSimplifierPassName)
+  explicit InstructionSimplifier(HGraph* graph,
+                                 OptimizingCompilerStats* stats = nullptr,
+                                 const char* name = kInstructionSimplifierPassName)
       : HOptimization(graph, name, stats) {}
 
   static constexpr const char* kInstructionSimplifierPassName = "instruction_simplifier";