Align code generation of local decl dimensions with top-level decls

[WardBrian]

Fixes the issue @nhuurre noted here where a local variable size check could be invalid in the presence of RNGs.

In particular, we had two independent code paths in the AST->MIR lowering, one for top-level (possibly transformed) decls, and one for local decls. They both did their own independent things in terms of avoiding generating redundant checks, how the names were constructed, etc.

By combining them, these differences go away and it naturally fixes the above issue by generating a temporary and using that as the size in the check and declaration.

Submission Checklist

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

Release notes

Improved code-generation of local variable dimensions.

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]

Codecov Report

❌ Patch coverage is 93.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.37%. Comparing base (689d502) to head (cc323ae).

Files with missing lines	Patch %	Lines
src/frontend/Ast_to_Mir.ml	92.45%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1691      +/-   ##
==========================================
- Coverage   92.42%   92.37%   -0.05%     
==========================================
  Files          69       69              
  Lines       10202    10188      -14     
==========================================
- Hits         9429     9411      -18     
- Misses        773      777       +4     

Files with missing lines	Coverage Δ
src/middle/Utils.ml	97.36% <100.00%> (+0.39%)	⬆️
src/stan_math_backend/Transform_Mir.ml	95.55% <100.00%> (ø)
src/frontend/Ast_to_Mir.ml	95.00% <92.45%> (-1.00%)	⬇️

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