Add ability to generate mock initial values, mirrors debug-generate-data

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [ ] If a user-facing facing change was made, the documentation PR is here:
  • [x] OR, no user-facing changes were made

Release notes

Adds a --debug-generate-inits flag to stanc which behaves like --debug-generate-data but for declarations in the parameters block.

This is useful for debugging in issues such as #1304

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[codecov[bot]]

Codecov Report

Merging #1306 (743e7ff) into master (cafcd69) will decrease coverage by 0.03%. The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master    #1306      +/-   ##
==========================================
- Coverage   89.06%   89.03%   -0.03%     
==========================================
  Files          64       64              
  Lines        9756     9760       +4     
==========================================
+ Hits         8689     8690       +1     
- Misses       1067     1070       +3     

Impacted Files	Coverage Δ
src/stanc/stanc.ml	80.15% <16.66%> (-1.81%)	:arrow_down:
...analysis_and_optimization/Debug_data_generation.ml	89.44% <100.00%> (ø)
src/frontend/Ast_to_Mir.ml	92.21% <100.00%> (ø)