3C JSON output does not escape backslashes in Windows file paths in string literals

[mattmccutchen-cci]

In the json_formating.c test on Windows, constraint_output.json_final_output.json contains text like this:

{"line":"C:\Users\correct\source\repos\checkedc-clang\clang\test\3C\json_formating.c:8:6:6","Variables":[{"PointerVar":{"Vars":["a_0"], "name":"a"}}]}

This is invalid JSON because of the unescaped backslashes in the file path. Kudos to #605 for detecting this bug, even if it makes more work for us to get the test passing on Windows before we can submit a PR to Microsoft.

Now, is it time to use a real JSON library to generate the files to hopefully prevent this kind of problem forever? The LLVM monorepo already contains its own JSON library; I ran across this when I was fixing the clangd3c build, because clangd uses JSON for the Language Server Protocol. Or we could try to manually fix only the fields we know are a problem now to unblock the submission to Microsoft and leave the churn of the full migration to the JSON library for later.

For convenience of testing the problem on Linux, I made a json-formatting-windows branch that changes the json_formating.c test to use a header file with a backslash in the name. (A backslash is an ordinary character in file paths on Linux, though parts of the LLVM codebase have trouble with it.) We probably can't merge this test to main in its current form because the backslash may cause problems with checking out the repository on Windows, but if we want to keep test coverage for the problem on Linux, we could have a test marked UNSUPPORTED: windows that uses a RUN command to create a file with a backslash at runtime. I'd be happy to write this test if it would be helpful.

[mattmccutchen-cci]

Per discussion with Mike, I'm going to do the partial fix to unblock the Windows test as soon as I can.

[mattmccutchen-cci]

I'm now using this issue to refer to the minimal fix to make the test pass on Windows. #620 is for the complete migration.