[ADT, CSSPGO] Specify set comparer

In _GLIBCXX_DEBUG builds, potentially implicitly enabled by
LLVM_ENABLE_EXPENSIVE_CHECKS, std::set<A, B>::iterator and
std::set<A, C>::iterator are distinct types that are not
interconvertible. This change aligns the iterator types with the set
types.

Reviewed By: hoy

Differential Revision: https://reviews.llvm.org/D119798