Cleanup warnings.

[trivialfis]

From clang-tidy-9 and gcc-7: Invalid case style, narrowing definition, wrong
initialization order, unused variables.

[codecov-io]

Codecov Report

Merging #5247 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5247   +/-   ##
=======================================
  Coverage   81.66%   81.66%           
=======================================
  Files          11       11           
  Lines        2389     2389           
=======================================
  Hits         1951     1951           
  Misses        438      438

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[hcho3]

Why are we removing num_elements from adapters?

[RAMitchell]

Its unused I think. Any way to make these checks part of CI? That way you won't have to cleanup after me in future :)

[hcho3]

@RAMitchell CI already runs clang-tidy, but an older version of it (7):

xgboost/tests/ci_build/Dockerfile.clang_tidy

Line 10 in ea850ec

apt-get install -y tar unzip wget git build-essential python3 python3-pip llvm-7 clang-tidy-7 clang-7 && \

Maybe we should upgrade clang-tidy to 9?

[RAMitchell]

Sounds good!

[trivialfis]

It requires adding latest llvm distribution to CI I believe. Will do that after the release.