[libc++] Implement generic associative container benchmarks

This patch implements generic associative container benchmarks
for containers with unique keys. In doing so, it replaces the
existing std::map benchmarks which were based on the cartesian
product infrastructure and were too slow to execute.

These new benchmarks aim to strike a balance between exhaustive
coverage of all operations in the most interesting case, while
executing fairly rapidly (~40s on my machine).

libcxx/test/benchmarks/GenerateInput.h

@@ -190,13 +190,18 @@ struct Generate<T> {
   static T arbitrary() { return 42; }
   static T cheap() { return 42; }
   static T expensive() { return 42; }
+  static T random() { return getRandomInteger<T>(std::numeric_limits<T>::min(), std::numeric_limits<T>::max()); }
 };
 
 template <>
 struct Generate<std::string> {
   static std::string arbitrary() { return "hello world"; }
   static std::string cheap() { return "small"; }
   static std::string expensive() { return std::string(256, 'x'); }
+  static std::string random() {
+    auto length = getRandomInteger<std::size_t>(1, 1024);
+    return getRandomString(length);
+  }
 };
 
 #endif // BENCHMARK_GENERATE_INPUT_H