Fix constness / references in weak ordering operator() signatures.

[bdice]

Resolves issues raised in #10730 about operator() being const and accepting references. cc: @ttnghia @jrhemstad

I applied const noexcept across all operator()s in the experimental row comparators, to match the existing pattern.

[codecov]

Codecov Report

Merging #10846 (3433ccc) into branch-22.06 (e0d94f3) will increase coverage by 0.03%.
The diff coverage is 100.00%.

❗ Current head 3433ccc differs from pull request most recent head 67ff2f5. Consider uploading reports for the commit 67ff2f5 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.06   #10846      +/-   ##
================================================
+ Coverage         86.28%   86.32%   +0.03%     
================================================
  Files               144      144              
  Lines             22654    22656       +2     
================================================
+ Hits              19548    19558      +10     
+ Misses             3106     3098       -8     

Impacted Files	Coverage Δ
python/cudf/cudf/core/resample.py	88.97% <ø> (ø)
python/dask_cudf/dask_cudf/groupby.py	97.42% <100.00%> (+0.01%)	⬆️
python/dask_cudf/dask_cudf/tests/test_groupby.py	100.00% <100.00%> (ø)
python/cudf/cudf/core/dataframe.py	93.78% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/column/string.py	88.78% <0.00%> (+0.12%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.79% <0.00%> (+0.22%)	⬆️
python/dask_cudf/dask_cudf/core.py	73.62% <0.00%> (+0.26%)	⬆️
python/cudf/cudf/core/column/numerical.py	96.17% <0.00%> (+0.29%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️
... and 1 more

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[jrhemstad]

@gpucibot merge