clang-tidy correctly run by CI

[henryiii]

Helping with #389, orthogonal changes. Clang-tidy now run by CI, warnings are shown. Will fix two errors as well in preparation for CLI11 1.9. CLANG_TIDY_FIX reworked into CLI11_CLANG_TIDY and CLI11_CLANG_TIDY_OPTIONS.

[cbachhuber]

Awesome, thanks a lot!

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@59052a6). Click here to learn what that means.
The diff coverage is 100%.

@@           Coverage Diff           @@
##             master   #390   +/-   ##
=======================================
  Coverage          ?   100%           
=======================================
  Files             ?     12           
  Lines             ?   3565           
  Branches          ?      0           
=======================================
  Hits              ?   3565           
  Misses            ?      0           
  Partials          ?      0

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <ø> (ø)
include/CLI/Validators.hpp	100% <100%> (ø)
include/CLI/Config.hpp	100% <100%> (ø)

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