CI: Add IWYU (include what you use) action.

[ni4]

Let's whether it would work at all.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.87%. Comparing base (ec78117) to head (5b0389a). Report is 38 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2283      +/-   ##
==========================================
- Coverage   84.88%   84.87%   -0.01%     
==========================================
  Files         114      114              
  Lines       22783    22783              
==========================================
- Hits        19339    19338       -1     
- Misses       3444     3445       +1     

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[ni4]

Closing this as it's usage for CI is controversial.