Update EntityKind

[Dalan94]

The CXCursor_TranslationUnit change value in LLVM 15 from 300 to 350 (see this commit).
The clang::EntityKind::TranslationUnit value is then wrong and a translation unit entity kind become NotImplemented.

[bonzini]

This is a mess; it's an ABI breakage. clang_15_0 should be introduced both in clang-sys and here, and the right value should be used depending on whether #[cfg(feature="clang_15_0")] or #[cfg(not(feature="clang_15_0"))].