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/nodes.h b/compiler/optimizing/nodes.h
index 7c3ca5c..caecc57 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -837,7 +837,7 @@
 
 class HBasicBlock : public ArenaObject<kArenaAllocBasicBlock> {
  public:
-  HBasicBlock(HGraph* graph, uint32_t dex_pc = kNoDexPc)
+  explicit HBasicBlock(HGraph* graph, uint32_t dex_pc = kNoDexPc)
       : graph_(graph),
         predecessors_(graph->GetArena()->Adapter(kArenaAllocPredecessors)),
         successors_(graph->GetArena()->Adapter(kArenaAllocSuccessors)),
@@ -6282,7 +6282,7 @@
 
 class HBoundType FINAL : public HExpression<1> {
  public:
-  HBoundType(HInstruction* input, uint32_t dex_pc = kNoDexPc)
+  explicit HBoundType(HInstruction* input, uint32_t dex_pc = kNoDexPc)
       : HExpression(Primitive::kPrimNot, SideEffects::None(), dex_pc),
         upper_bound_(ReferenceTypeInfo::CreateInvalid()) {
     SetPackedFlag<kFlagUpperCanBeNull>(true);