[clang] [-Wcompletion-handler] Fix a non-termination issue (PR #78380)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

@SavchenkoValeriy, sorry for the late response.  
Your suggested fix works for my minimal example as well as my local project where this bug was originated.
And I failed to come up with a new counter-example for the fix so I believe it will also prevent future non-termination issues. 
I have updated the PR with respect to the simpler fix.

https://github.com/llvm/llvm-project/pull/78380