[BOLT][NFC] Simplify code using std::optional

[kongyi]

Summary

Use std::optional instead of tracking if it is the first profile seen.

bolt/tools/merge-fdata/merge-fdata.cpp

bool isYAML(const StringRef Filename) {
   StringRef Buffer = MB.get()->getBuffer();
   if (Buffer.startswith("---\n"))
     return true;
   return false;
 }
 
 void mergeLegacyProfiles(const SmallVectorImpl<std::string> &Filenames) {
   errs() << "Using legacy profile format.\n";
-  bool BoltedCollection = false;
-  bool First = true;
+  std::optional<bool> BoltedCollection;
   StringMap<uint64_t> Entries;
   for (const std::string &Filename : Filenames) {
     if (isYAML(Filename))
       report_error(Filename, "cannot mix YAML and legacy formats");
     ErrorOr<std::unique_ptr<MemoryBuffer>> MB =
         MemoryBuffer::getFileOrSTDIN(Filename);
     if (std::error_code EC = MB.getError())
       report_error(Filename, EC);
     errs() << "Merging data from " << Filename << "...\n";
 
     StringRef Buf = MB.get()->getBuffer();
     // Check if the string "boltedcollection" is in the first line
     if (Buf.startswith("boltedcollection\n")) {
-      if (!First && !BoltedCollection)
+      if (!BoltedCollection.value_or(true))
         report_error(
             Filename,
             "cannot mix profile collected in BOLT and non-BOLT deployments");
       BoltedCollection = true;
       Buf = Buf.drop_front(17);
     } else {
-      if (BoltedCollection)
+      if (BoltedCollection.value_or(false))
         report_error(
             Filename,
             "cannot mix profile collected in BOLT and non-BOLT deployments");
+      BoltedCollection = false;
     }
 
     SmallVector<StringRef> Lines;
     SplitString(Buf, Lines, "\n");
     for (StringRef Line : Lines) {
       size_t Pos = Line.rfind(" ");
       if (Pos == StringRef::npos)
         report_error(Filename, "Malformed / corrupted profile");
       StringRef Signature = Line.substr(0, Pos);
       uint64_t Count;
       if (Line.substr(Pos + 1, Line.size() - Pos).getAsInteger(10, Count))
         report_error(Filename, "Malformed / corrupted profile counter");
       Count += Entries.lookup(Signature);
       Entries.insert_or_assign(Signature, Count);
     }
-    First = false;
   }
 
   if (BoltedCollection)
     output() << "boltedcollection\n";
   for (const auto &Entry : Entries)
     output() << Entry.getKey() << " " << Entry.getValue() << "\n";
 
   errs() << "Profile from " << Filenames.size() << " files merged.\n";