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