[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).

Author: ldionne

libcxx/test/benchmarks/GenerateInput.h

@@ -11,6 +11,7 @@
 
 #include <algorithm>
 #include <climits>
+#include <concepts>
 #include <cstddef>
 #include <random>
 #include <string>
@@ -171,4 +172,36 @@ inline std::vector<const char*> getRandomCStringInputs(std::size_t N) {
   return cinputs;
 }
 
+template <class T>
+struct Generate {
+  // When the contents don't matter
+  static T arbitrary();
+
+  // Prefer a cheap-to-construct element if possible
+  static T cheap();
+
+  // Prefer an expensive-to-construct element if possible
+  static T expensive();
+};
+
+template <class T>
+  requires std::integral<T>
+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