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

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

> Do you still want an issue to be opened?

I sent out a follow-up PR at https://github.com/llvm/llvm-project/pull/118906 to add a comment describing the situations in which `IdxContents` is used, to avoid future confusion.

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