Support: Avoid unnecessary std::function for SignpostEmitterImpl::SignpostLog

[dexonsmith]

Summary

The destructor for SignPostEmitterImpl::SignpostLog is known statically. Avoid
the unnecessary vtable indirection through std::function in the std::unique_ptr
by turning LogDeleter into a struct. No real functionality change here.

[dsanders]

LGTM

llvm/lib/Support/Signposts.cpp

 #define SIGNPOSTS_AVAILABLE()                                                  \
   __builtin_available(macos 10.14, iOS 12, tvOS 12, watchOS 5, *)
 namespace {
 os_log_t *LogCreator() {
   os_log_t *X = new os_log_t;
   *X = os_log_create("org.llvm.signposts", OS_LOG_CATEGORY_POINTS_OF_INTEREST);
   return X;
 }
-void LogDeleter(os_log_t *X) {
+struct LogDeleter {
+  void operator()(os_log_t *X) const {
     os_release(*X);
     delete X;
   }
+};
 } // end anonymous namespace
 
 namespace llvm {
 class SignpostEmitterImpl {
-  using LogPtrTy = std::unique_ptr<os_log_t, std::function<void(os_log_t *)>>;
+  using LogPtrTy = std::unique_ptr<os_log_t, LogDeleter>;
   using LogTy = LogPtrTy::element_type;
 
   LogPtrTy SignpostLog;
   DenseMap<const void *, os_signpost_id_t> Signposts;
   sys::SmartMutex<true> Mutex;
 
   LogTy &getLogger() const { return *SignpostLog; }
   os_signpost_id_t getSignpostForObject(const void *O) {
     sys::SmartScopedLock<true> Lock(Mutex);
     const auto &I = Signposts.find(O);
     if (I != Signposts.end())
       return I->second;
     os_signpost_id_t ID = {};
     if (SIGNPOSTS_AVAILABLE()) {
       ID = os_signpost_id_make_with_pointer(getLogger(), O);
     }
     const auto &Inserted = Signposts.insert(std::make_pair(O, ID));
     return Inserted.first->second;
   }
 
 public:
-  SignpostEmitterImpl() : SignpostLog(LogCreator(), LogDeleter) {}
+  SignpostEmitterImpl() : SignpostLog(LogCreator()) {}
 
   bool isEnabled() const {
     if (SIGNPOSTS_AVAILABLE())
       return os_signpost_enabled(*SignpostLog);
     return false;
   }
 
   void startInterval(const void *O, llvm::StringRef Name) {