[ELF] - Never use std::sort.

It turns out we should not use the std::sort anymore.
r327219 added a new wrapper llvm::sort (D39245).
When EXPENSIVE_CHECKS is defined, it shuffles the
input container and that helps to find non-deterministic
ordering.

Patch changes code to use llvm::sort and std::stable_sort
instead of std::sort

Differential revision: https://reviews.llvm.org/D45969

llvm-svn: 330702