[clang-tools-extra] [clangd] Explicitly block until async task completes (PR #130077)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov approved this pull request.

LGTM.

PS it would've probably not been my first choice to fix this at the use site rather than at the declaration of the type itself. I am worried that someone else using this elsewhere may end up running into the same problem. 
However, I don't have strong opinions on it, so approving anyway. Just wanted to hear the rationale you have in mind for doing it at call site rather than defining a destructor that waits.

https://github.com/llvm/llvm-project/pull/130077