[clang] Reformat cindex. NFC.

to reduce spurios changes in patches after clang-formatting them. In
particular, these files contain long enums that clang-format reformats
in their entirety if e.g. an element is added.

Reviews having this problem include https://reviews.llvm.org/D76342 and
https://reviews.llvm.org/D71447.

Author: Meinersbur