[instrprof][NFC] Templatize the instrprof iterator.

[snehasish]

Summary

This change templatizes the InstrProfIterator where the default
specialization is based on the current usage, i.e. the reader_type is
InstrProfReader and the record_type (value_type) is
NamedInstrProfRecord. A subsequent patch will use the same iterator
template to implement an iterator for the RawMemProfReader.

[davidxl]

lgtm

llvm/include/llvm/ProfileData/InstrProfReader.h

 #include <utility>
 #include <vector>
 
 namespace llvm {
 
 class InstrProfReader;
 
 /// A file format agnostic iterator over profiling data.
+template <class record_type = NamedInstrProfRecord,
+          class reader_type = InstrProfReader>
 class InstrProfIterator {
 public:
   using iterator_category = std::input_iterator_tag;
-  using value_type = NamedInstrProfRecord;
+  using value_type = record_type;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;
   using reference = value_type &;
 
 private:
-  InstrProfReader *Reader = nullptr;
+  reader_type *Reader = nullptr;
   value_type Record;
 
-  void Increment();
+  void increment() {
+    if (Error E = Reader->readNextRecord(Record)) {
+      // Handle errors in the reader.
+      InstrProfError::take(std::move(E));
+      *this = InstrProfIterator();
+    }
+  }
 
 public:
   InstrProfIterator() = default;
-  InstrProfIterator(InstrProfReader *Reader) : Reader(Reader) { Increment(); }
+  InstrProfIterator(reader_type *Reader) : Reader(Reader) { increment(); }
 
-  InstrProfIterator &operator++() { Increment(); return *this; }
+  InstrProfIterator &operator++() {
+    increment();
+    return *this;
+  }
   bool operator==(const InstrProfIterator &RHS) const {
     return Reader == RHS.Reader;
   }
   bool operator!=(const InstrProfIterator &RHS) const {
     return Reader != RHS.Reader;
   }
   value_type &operator*() { return Record; }
   value_type *operator->() { return &Record; }
public:
 
   /// Read a single record.
   virtual Error readNextRecord(NamedInstrProfRecord &Record) = 0;
 
   /// Print binary ids on stream OS.
   virtual Error printBinaryIds(raw_ostream &OS) { return success(); };
 
   /// Iterator over profile data.
-  InstrProfIterator begin() { return InstrProfIterator(this); }
-  InstrProfIterator end() { return InstrProfIterator(); }
+  InstrProfIterator<> begin() { return InstrProfIterator<>(this); }
+  InstrProfIterator<> end() { return InstrProfIterator<>(); }
 
   virtual bool isIRLevelProfile() const = 0;
 
   virtual bool hasCSIRLevelProfile() const = 0;
 
   virtual bool instrEntryBBEnabled() const = 0;
 
   /// Return true if we must provide debug info to create PGO profiles.

llvm/lib/ProfileData/InstrProfReader.cpp

IndexedInstrProfReader::create(std::unique_ptr<MemoryBuffer> Buffer,
 
   // Initialize the reader and return the result.
   if (Error E = initializeReader(*Result))
     return std::move(E);
 
   return std::move(Result);
 }
 
-void InstrProfIterator::Increment() {
-  if (auto E = Reader->readNextRecord(Record)) {
-    // Handle errors in the reader.
-    InstrProfError::take(std::move(E));
-    *this = InstrProfIterator();
-  }
-}
-
 bool TextInstrProfReader::hasFormat(const MemoryBuffer &Buffer) {
   // Verify that this really looks like plain ASCII text by checking a
   // 'reasonable' number of characters (up to profile magic size).
   size_t count = std::min(Buffer.getBufferSize(), sizeof(uint64_t));
   StringRef buffer = Buffer.getBufferStart();
   return count == 0 ||
          std::all_of(buffer.begin(), buffer.begin() + count,
                      [](char c) { return isPrint(c) || isSpace(c); });