[PM] Make PassManager's constructor explicit.

[silvas]

This type of cleanup can be done with post commit review btw.

llvm/trunk/include/llvm/IR/PassManager.h

template <typename IRUnitT,
           typename AnalysisManagerT = AnalysisManager<IRUnitT>,
           typename... ExtraArgTs>
 class PassManager : public PassInfoMixin<
                         PassManager<IRUnitT, AnalysisManagerT, ExtraArgTs...>> {
 public:
   /// \brief Construct a pass manager.
   ///
   /// It can be passed a flag to get debug logging as the passes are run.
-  PassManager(bool DebugLogging = false) : DebugLogging(DebugLogging) {}
+  explicit PassManager(bool DebugLogging = false) : DebugLogging(DebugLogging) {}
+
   // FIXME: These are equivalent to the default move constructor/move
   // assignment. However, using = default triggers linker errors due to the
   // explicit instantiations below. Find away to use the default and remove the
   // duplicated code here.
   PassManager(PassManager &&Arg)
       : Passes(std::move(Arg.Passes)),
         DebugLogging(std::move(Arg.DebugLogging)) {}
   PassManager &operator=(PassManager &&RHS) {