ast-exporter: pass compilation database directory to Clang

[ShiroKSH]

Fixes #1809.

CommonOptionsParser expects -p to name the build directory, but the AST exporter passed compile_commands.json itself. Pass its parent directory instead, using . for a bare relative path, and cover both forms.

This is a general correctness fix; #1809 exposed the existing mismatch on Clang 22.

[thedataking]

CommonOptionsParser treats -p as a build directory and searches that directory for the compilation database. The AST exporter passed the JSON file itself, so newer Clang versions attempted to treat the file as a directory and continued without the database flags.

https://github.com/llvm/llvm-project/commits/main/clang/lib/Tooling/CommonOptionsParser.cpp has not changed its functionality since 2022. I also don't see any recent changes to https://github.com/llvm/llvm-project/commits/main/clang/lib/Tooling/CompilationDatabase.cpp that would make this a clang 22 specific issue.

I do agree that we ought to pass the parent directory of the compilation database, so that part looks good to me. However, I think we ought to clarify whether this is a general correctness fix or what versions of clang need this.

nitpick: C2Rust validation means passing CI so we do not need a validation section. Generally, PRs bodies are shorter and not split into sections. I'd encourage you to take a look at other PRs and make yours follow project conventions.

[ShiroKSH]

Thanks — I shortened the PR body to match the project convention and clarified that this is a general correctness fix; #1809 only exposed the existing mismatch on Clang 22. I also rebased the branch onto the current master.

[thedataking]

Thanks!