Improve code generation and error wording around top-level decl sizes

[WardBrian]

Closes #1646 (by improving the error message and some code gen, not by actually implementing the request)

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 error messages for when a variable has a size which depends on an illegal expression.

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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.51%. Comparing base (5496e41) to head (c513265).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1647   +/-   ##
=======================================
  Coverage   91.51%   91.51%           
=======================================
  Files          67       67           
  Lines        9977     9975    -2     
=======================================
- Hits         9130     9129    -1     
+ Misses        847      846    -1     

Files with missing lines	Coverage Δ
src/frontend/Ast_to_Mir.ml	96.00% <100.00%> (-0.01%)	⬇️
src/frontend/Semantic_error.ml	96.19% <100.00%> (+0.01%)	⬆️
src/frontend/Typechecker.ml	94.87% <100.00%> (ø)
src/stan_math_backend/Lower_program.ml	99.19% <100.00%> (-0.01%)	⬇️

... and 1 file with indirect coverage changes

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