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

[Vakhurin Sergei via cfe-commits]

igelbox wrote:

> Have you run your changes with address and ub sanitizers to see if there are some uncaught edge cases?

Not quire sure I get the point about UB-sanitizers. I tested this changes against my proprietary code which caused the issue with OOM/infinite-loop and it works just fine. I can and will build the `clangd` with assertions enabled and test it against my codebase to try catching other cases.

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