Clear lexing stack before parsing starts

[WardBrian]

This fixes an issue first spotted by @spinkney in #577. The preprocessing stack is never cleared, so in particular a subsequent parse will inherit anything left on the stack. This is only possible to trigger in the Javascript compiler.

Removing the change in Preprocessor.ml makes the new test fail like so:

Syntax error in 'string', line 8, column 0 to column 5, parsing error:
Expected "generated quantities {" or end of file after end of model block.

Syntax error in 'string', line 8, column 6 to column 7, parsing error:
Expected "generated quantities {" or end of file after end of model block.

Submission Checklist

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

Release notes

Fix an issue with parser errors 'sticking around' on subsequent runs, which primarily affected the Javascript compiler

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]

I believe this behavior can/has affected more recent versions of RStan -- see, for example, this comment. It has certainly been present in basically every version of the javascript compiler, but it's tricky to trigger unintentionally.

[spinkney]

was this found by seantalts?

[WardBrian]

This is my third time this week tagging the wrong GitHub username. I don’t know what’s gotten into me

[rok-cesnovar]

Nice catch! Thanks!