[llvm] [ADT][ProfileData] Introduce SortedVectorMap and switch CallTargetMap to it (PR #215733)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This is similar to Boost.Container's flat_map (AoS), not `std::flat_map` (SoA). Not sure how `std::flat_map` made it into the standard... it is niche and `flat` doesn't reflect the semantics well. I think `SortedVectorMap` is ok, since it will probably not be used much.

https://github.com/llvm/llvm-project/pull/215733