[ELF] Use union-find set and doubly linked list in Call-Chain Clustering (C³) heuristic

Before, SecToClusters[*] was used to track the belonged cluster.
During a merge (From -> Into), every element of From has to be updated.
Use a union-find set to speed up this use case.

Also, replace `std::vector<int> Sections;` with a doubly-linked
pointers: int Next, Prev;

Reviewed By: Bigcheese

Differential Revision: https://reviews.llvm.org/D46228

llvm-svn: 373708