Fixes complexity of map insert_or_assign with a hint.

Mitsuru Kariya reported the map operations insert_or_assign with a hint
violates the complexity requirement. The function no longer uses a lower_bound,
which caused the wrong complexity.

Fixes PR38722: [C++17] std::map::insert_or_assign w/ hint violate complexity requirements

Differential Revision: https://reviews.llvm.org/D62779