Support: Drop the no-op initializer for SignpostEmitterImpl::Signposts, NFC

[dexonsmith]

Summary

This is a DenseMap, which has its own initializer; we don't need to explicitly
call the default constructor here.

[dsanders]

LGTM

llvm/lib/Support/Signposts.cpp

os_signpost_id_t getSignpostForObject(const void *O) {
     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), Signposts() {}
+  SignpostEmitterImpl() : SignpostLog(LogCreator(), LogDeleter) {}
 
   bool isEnabled() const {
     if (SIGNPOSTS_AVAILABLE())
       return os_signpost_enabled(*SignpostLog);
     return false;
   }
 
   void startInterval(const void *O, llvm::StringRef Name) {