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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> idxcontents is only used when we have a set of file names available. hence this was only set via relevant constructors and rest of the runtime doesn't use the field.
> 
> but we don't construct tons of new index instances every second. so it's totally fine to just initialize them to `none`

Thanks for the explanation.

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