Use -fsyntax-only in Jenkins

[WardBrian]

Submission Checklist

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

Release notes

Try to use -fsyntax-only argument to Clang to speed up CI during the phases that only check if something actually compiles.

-fsyntax-only

Run the preprocessor, parser and semantic analysis stages

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)

[WardBrian]

This shaves ~20-30 minutes off of the CI compared to what we do right now. If we don't actually care about code-generating or linking of the stanc-generated code, it seems like a nice speedup