[NFC] complexity_n is not of IterationCount type

There is no bug here, but it gave me a scare the other day.
It is not incorrect to use `IterationCount` here,
since it's just an `int64_t` either way,
but it's wildly confusing. Let's not do that.

include/benchmark/benchmark.h

@@ -672,13 +672,15 @@ typedef std::map<std::string, Counter> UserCounters;
 // calculated automatically to the best fit.
 enum BigO { oNone, o1, oN, oNSquared, oNCubed, oLogN, oNLogN, oAuto, oLambda };
 
+typedef int64_t ComplexityN;
+
 typedef int64_t IterationCount;
 
 enum StatisticUnit { kTime, kPercentage };
 
 // BigOFunc is passed to a benchmark in order to specify the asymptotic
 // computational complexity for the benchmark.
-typedef double(BigOFunc)(IterationCount);
+typedef double(BigOFunc)(ComplexityN);
 
 // StatisticsFunc is passed to a benchmark in order to compute some descriptive
 // statistics over all the measurements of some type
@@ -875,10 +877,12 @@ class BENCHMARK_EXPORT State {
   // and complexity_n will
   // represent the length of N.
   BENCHMARK_ALWAYS_INLINE
-  void SetComplexityN(int64_t complexity_n) { complexity_n_ = complexity_n; }
+  void SetComplexityN(ComplexityN complexity_n) {
+    complexity_n_ = complexity_n;
+  }
 
   BENCHMARK_ALWAYS_INLINE
-  int64_t complexity_length_n() const { return complexity_n_; }
+  ComplexityN complexity_length_n() const { return complexity_n_; }
 
   // If this routine is called with items > 0, then an items/s
   // label is printed on the benchmark report line for the currently
@@ -967,7 +971,7 @@ class BENCHMARK_EXPORT State {
   // items we don't need on the first cache line
   std::vector<int64_t> range_;
 
-  int64_t complexity_n_;
+  ComplexityN complexity_n_;
 
  public:
   // Container for user-defined counters.
@@ -1805,7 +1809,7 @@ class BENCHMARK_EXPORT BenchmarkReporter {
     // Keep track of arguments to compute asymptotic complexity
     BigO complexity;
     BigOFunc* complexity_lambda;
-    int64_t complexity_n;
+    ComplexityN complexity_n;
 
     // what statistics to compute from the measurements
     const std::vector<internal::Statistics>* statistics;

src/complexity.cc

@@ -77,12 +77,12 @@ std::string GetBigOString(BigO complexity) {
 // given by the lambda expression.
 //   - n             : Vector containing the size of the benchmark tests.
 //   - time          : Vector containing the times for the benchmark tests.
-//   - fitting_curve : lambda expression (e.g. [](int64_t n) {return n; };).
+//   - fitting_curve : lambda expression (e.g. [](ComplexityN n) {return n; };).
 
 // For a deeper explanation on the algorithm logic, please refer to
 // https://en.wikipedia.org/wiki/Least_squares#Least_squares,_regression_analysis_and_statistics
 
-LeastSq MinimalLeastSq(const std::vector<int64_t>& n,
+LeastSq MinimalLeastSq(const std::vector<ComplexityN>& n,
                        const std::vector<double>& time,
                        BigOFunc* fitting_curve) {
   double sigma_gn_squared = 0.0;
@@ -124,7 +124,7 @@ LeastSq MinimalLeastSq(const std::vector<int64_t>& n,
 //   - complexity : If different than oAuto, the fitting curve will stick to
 //                  this one. If it is oAuto, it will be calculated the best
 //                  fitting curve.
-LeastSq MinimalLeastSq(const std::vector<int64_t>& n,
+LeastSq MinimalLeastSq(const std::vector<ComplexityN>& n,
                        const std::vector<double>& time, const BigO complexity) {
   BM_CHECK_EQ(n.size(), time.size());
   BM_CHECK_GE(n.size(), 2);  // Do not compute fitting curve is less than two
@@ -164,7 +164,7 @@ std::vector<BenchmarkReporter::Run> ComputeBigO(
   if (reports.size() < 2) return results;
 
   // Accumulators.
-  std::vector<int64_t> n;
+  std::vector<ComplexityN> n;
   std::vector<double> real_time;
   std::vector<double> cpu_time;