Use std::less instead of operator < in less_first and less_second

[ahatanak]

Summary

According to the standard, if p1 and p2 are both pointers, p1 < p2 and p2 < p1 can both be false in theory in some cases:

https://eel.is/c++draft/expr.rel#4.3

std::less<void> yields a implementation-defined strict total order over pointers:

https://eel.is/c++draft/comparisons.general

[ldionne]

I don't know that it makes a difference in practice, but this definitely LGTM on principle.

llvm/include/llvm/ADT/STLExtras.h

 //===----------------------------------------------------------------------===//
 //     Extra additions to <utility>
 //===----------------------------------------------------------------------===//
 
 /// Function object to check whether the first component of a std::pair
 /// compares less than the first component of another std::pair.
 struct less_first {
   template <typename T> bool operator()(const T &lhs, const T &rhs) const {
-    return lhs.first < rhs.first;
+    return std::less<>()(lhs.first, rhs.first);
   }
 };
 
 /// Function object to check whether the second component of a std::pair
 /// compares less than the second component of another std::pair.
 struct less_second {
   template <typename T> bool operator()(const T &lhs, const T &rhs) const {
-    return lhs.second < rhs.second;
+    return std::less<>()(lhs.second, rhs.second);
   }
 };
 
 /// \brief Function object to apply a binary function to the first component of
 /// a std::pair.
 template<typename FuncTy>
 struct on_first {
   FuncTy func;