build and test LLVM version 16

[JakeHillion]

Add LLVM-16 to the Nix builds and CI matrix. Also add the option to explicitly select an LLVM version so the CircleCI build can still work as before.

We should support at least LLVM-17 currently, but there appears to be a significant performance regression in compiling the generated code for large variants that I'm going to debug separately. The long term goal is to support and test all versions 15 and up.

Test plan:

• CI

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 60.79%. Comparing base (40d3f39) to head (eabe363). Report is 23 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #504   +/-   ##
=======================================
  Coverage   60.79%   60.79%           
=======================================
  Files         126      126           
  Lines       12493    12493           
  Branches     2014     2014           
=======================================
  Hits         7595     7595           
  Misses       3941     3941           
  Partials      957      957           

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