[clang-tools-extra] [clangd] Test background index file contents (PR #179956)

[Nathan Ridge via cfe-commits]

HighCommander4 wrote:

I'm sorry, I'm quite overloaded with reviews at the moment and thus unlikely to get to this for a while. Adding some other reviewers in the hope they might get to it before me.

@ArcsinX, if you're comfortable approving / not approving the patch without my involvement, please go ahead.

https://github.com/llvm/llvm-project/pull/179956