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

[kadir çetinkaya via cfe-commits]

kadircet 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`

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