[clang] [clang-tools-extra] Fix OOM in FormatDiagnostic (PR #108187)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> > LGTM! Do you need someone to land these changes on your behalf?
> 
> I suppose so.. or what are my options? The Merge button isn't active of course. I believe someone from maintainers would merge this PR and I will be so proud)

I've merged on your behalf (I sometime struggle to tell who has commit privileges and who doesn't), thank you!

https://github.com/llvm/llvm-project/pull/108187