Support multiple libclang versions

For a `\`, it might be better to modify the test to accept both, rather than forcing a newer version of clang for all users.

_Originally posted by @dcharkes in https://github.com/dart-lang/native/issues/255

It can be hard getting different libclang versions. See for example:

* https://github.com/flutter/flutter/issues/105513

We should support an as wide range of libclang as possible.

Currently, a single test fails with older versions of libclang due to the single `\`. We should just special-case that test to accept both versions.

* [x] fix test
* [ ] test both libclang 9 and the newest libclang on CI