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

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

Differential Revision: https://reviews.llvm.org/D108733

llvm/include/llvm/ADT/STLExtras.h

@@ -1273,7 +1273,7 @@ template <typename ContainerTy> auto make_second_range(ContainerTy &&c) {
 /// 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);
   }
 };
 
@@ -1281,7 +1281,7 @@ struct less_first {
 /// 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);
   }
 };