[examples] Move loop unroller passes into anonymous namespace.

[ChrisCummins]

This appears to be the way that the LLVM sources structure their
passes, with the implementation in an anonymous namespace and the
INITIALIZE_PASS_xxx macros in no namespace.

Fixes #509.

[codecov-commenter]

Codecov Report

Merging #523 (95b6e0f) into development (398b39f) will decrease coverage by 0.31%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #523      +/-   ##
===============================================
- Coverage        87.38%   87.06%   -0.32%     
===============================================
  Files              113      113              
  Lines             6411     6411              
===============================================
- Hits              5602     5582      -20     
- Misses             809      829      +20     

Impacted Files	Coverage Δ
compiler_gym/views/observation.py	97.29% <0.00%> (-2.71%)	⬇️
compiler_gym/envs/compiler_env.py	87.32% <0.00%> (-2.16%)	⬇️
compiler_gym/envs/llvm/compute_observation.py	87.50% <0.00%> (-2.09%)	⬇️
compiler_gym/util/runfiles_path.py	89.28% <0.00%> (-1.79%)	⬇️
compiler_gym/service/connection.py	75.67% <0.00%> (-1.36%)	⬇️
compiler_gym/views/observation_space_spec.py	90.52% <0.00%> (-1.06%)	⬇️
compiler_gym/envs/llvm/llvm_benchmark.py	89.92% <0.00%> (-0.78%)	⬇️
compiler_gym/datasets/dataset.py	84.96% <0.00%> (-0.76%)	⬇️
...loop_tool/service/loop_tool_compilation_session.py	88.59% <0.00%> (-0.68%)	⬇️

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[mostafaelhoushi]

LGTM