[llvm] [CodeLayout] CDSortImpl: remove HotChains and remove linear-time erase_value from mergeChains (PR #69276)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Thanks for the review!

This is not a bottleneck.
```
  Time (mean ± σ):     534.928 s ±  1.463 s    [User: 545.663 s, System: 2.177 s]
  Range (min … max):   533.894 s … 535.963 s    2 runs

Benchmark 2: numactl -C 20-27 /t/lld1 -flavor gnu @response.txt --call-graph-profile-sort=cdsort --threads=8
  Time (mean ± σ):     535.081 s ±  0.563 s    [User: 545.860 s, System: 2.193 s]
  Range (min … max):   534.683 s … 535.479 s    2 runs

Summary
  'numactl -C 20-27 /t/lld0 -flavor gnu @response.txt --call-graph-profile-sort=cdsort --threads=8' ran
    1.00 ± 0.00 times faster than 'numactl -C 20-27 /t/lld1 -flavor gnu @response.txt --call-graph-profile-sort=cdsort --threads=8'
```


https://github.com/llvm/llvm-project/pull/69276