Test various parts of the new code-gen

[WardBrian]

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] OR, no user-facing changes were made

Release notes

Added tests for various parts of the C++ backend which weren't previously tested, and added a --debug-lir flag to print out the C++ representation in a s-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[bot]]

Codecov Report

Merging #1276 (82fb5a1) into master (c6f83de) will increase coverage by 0.88%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1276      +/-   ##
==========================================
+ Coverage   87.93%   88.81%   +0.88%     
==========================================
  Files          64       64              
  Lines        9665     9668       +3     
==========================================
+ Hits         8499     8587      +88     
+ Misses       1166     1081      -85     

Impacted Files	Coverage Δ
src/middle/Stmt.ml	72.86% <ø> (+2.71%)	:arrow_up:
src/stan_math_backend/Lower_expr.ml	92.61% <100.00%> (+2.44%)	:arrow_up:
src/stan_math_backend/Lower_functions.ml	100.00% <100.00%> (+0.50%)	:arrow_up:
src/stanc/stanc.ml	84.55% <100.00%> (+0.25%)	:arrow_up:
src/stan_math_backend/Lower_stmt.ml	96.84% <0.00%> (+0.52%)	:arrow_up:
src/stan_math_backend/Cpp.ml	86.52% <0.00%> (+18.86%)	:arrow_up: