Fix clang-tidy related if statements with branch clone

[riemass]

Fixing if statements regarding bugprone-branch-clone.
I've disabled the warnings for further use since it gets in the way a lot.
To avoid merge conflicts, this PR also disables bugprone-forward-declaration-namespace. This complains a lot because of fwd.hpp style headers.
Pulled from #1565.
Relates #1435.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (7753180) 66.93% compared to head (9c45785) 66.83%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1576      +/-   ##
==========================================
- Coverage   66.93%   66.83%   -0.11%     
==========================================
  Files         644      644              
  Lines       27543    27549       +6     
  Branches     2924     2926       +2     
==========================================
- Hits        18437    18411      -26     
- Misses       7613     7641      +28     
- Partials     1493     1497       +4     

Files	Coverage Δ
libcaf_core/caf/config_value.hpp	67.94% <ø> (-0.21%)	⬇️
libcaf_core/caf/detail/glob_match.cpp	22.11% <ø> (ø)
...bcaf_core/caf/detail/stringification_inspector.hpp	94.38% <ø> (ø)
libcaf_core/caf/scheduled_actor.cpp	76.35% <100.00%> (+0.04%)	⬆️

... and 16 files with indirect coverage changes

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