Automatically detect libclang version when testing and use appropriate expectation files

[fitzgen]

Right now, we rely on the testing_only_libclang_X features to choose the correct expectation file for tests that have different output across libclang versions.

If none of those features are passed, then we should make an attempt to parse the libclang version and use those expectations, if possible.

I always forget to pass the testing_only_libclang_X feature when invoking cargo test and I expect others do too, so this would be a nice papercut fix.

We have a clang_version function already, we just need to check it in the compare_generated_header function in tests/test.rs.

If someone wants to work on this as their first bindgen issue, I can help mentor :)

[highfive]

Hi! If you have any questions regarding this issue, feel free to make a comment here, or ask it in the #servo channel in IRC.

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[tz70s]

@highfive: assign me
I'm interested in working on this issue!

[highfive]

Hey @tz70s! Thanks for your interest in working on this issue. It's now assigned to you!

[tz70s]

Hi @fitzgen !

Sorry, I should ask for your opinion on working this issue first. I have few questions about it after pulling requests.

1. The LLVM version 3.7 is using libclang-3.8 now, should I also add the auto transfer to it?
2. How about modify libclang-4 to libclang-4.0? This may reduce the condition branches and being compatible while if libclang-4.1 appeared.

thanks!