[clang-tools-extra] [NFC] Fix uninitialized scalar field in constructor. (PR #118324)

[Tom Honermann via cfe-commits]

https://github.com/tahonermann requested changes to this pull request.

How did you decide on `IndexContents::All` as the default initialization value. I would be more inclined towards `IndexContents::None`.

I think it would be preferred to use a member initializer in the declaration of `IdxContents`. That will ensure an initialization is performed if additional constructors are added.

https://github.com/llvm/llvm-project/pull/118324