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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> Seems like `MemIndex::IdxContents` has the same issue.
> 
> It's a bit suspicious to me that the other `DexIndex` and `MemIndex` constructors are not passing in an `IndexContents` value. Either there's a bug lurking there, or the field is only relevant in certain situations (e.g. only for dynamic indexes) in which case that should be documented at the field's declaration. Would you mind filing a follow-up issue about auditing this situation more carefully, so it doesn't get forgotten?

I could also initialize all the others to `None` and see if this doesn't break any tests?

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