Make constructors, that can be called with a single argument, explicit.

Also, fix two instances of unnecessary copy.

#xls-build-gardener

PiperOrigin-RevId: 721935924

xls/data_structures/leaf_type_tree.h

@@ -676,7 +676,8 @@ class SharedLeafTypeTree {
  private:
   std::variant<LeafTypeTreeView<T>, LeafTypeTree<T>> inner_;
 
-  SharedLeafTypeTree(std::variant<LeafTypeTreeView<T>, LeafTypeTree<T>>&& inner)
+  explicit SharedLeafTypeTree(
+      std::variant<LeafTypeTreeView<T>, LeafTypeTree<T>>&& inner)
       : inner_(std::move(inner)) {}
 
   friend class LeafTypeTree<T>;

xls/passes/dataflow_graph_analysis.h

@@ -39,8 +39,8 @@ namespace xls {
 // terms of the nodes emitting them) that fully determine the value of the node.
 class DataflowGraphAnalysis {
  public:
-  DataflowGraphAnalysis(FunctionBase* f,
-                        const QueryEngine* query_engine = nullptr);
+  explicit DataflowGraphAnalysis(FunctionBase* f,
+                                 const QueryEngine* query_engine = nullptr);
 
   absl::StatusOr<std::vector<Node*>> GetMinCutFor(
       Node* node, std::optional<int64_t> max_unknown_bits = std::nullopt,

xls/passes/pipeline_generator.h

@@ -123,7 +123,7 @@ class PipelineGeneratorBase {
                                element.options());
     }
     if (element.has_fixedpoint()) {
-      auto fp_proto = element.fixedpoint();
+      const auto& fp_proto = element.fixedpoint();
       auto fixedpoint =
           std::make_unique<FixedPointCompoundPassBase<IrT, OptionsT, ResultsT>>(
               fp_proto.has_short_name() ? fp_proto.short_name() : "fixedpoint",
@@ -138,7 +138,7 @@ class PipelineGeneratorBase {
       return absl::OkStatus();
     }
     if (element.has_pipeline()) {
-      auto p_proto = element.pipeline();
+      const auto& p_proto = element.pipeline();
       auto pipeline =
           std::make_unique<CompoundPassBase<IrT, OptionsT, ResultsT>>(
               p_proto.has_short_name() ? p_proto.short_name() : "pipeline",