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

[Vakhurin Sergei via cfe-commits]

igelbox wrote:

> I'm not certain that there is a reasonable way to add test coverage for the changes, but if you can devise a test, that would be appreciated.

I wonder there should be some tests like:
- input = some source files
- output = expected it compiles well and has no assertions fired

but not sure yet where to find such a tests-folder. I'm just a newbie in clang development workflow, so I wonder is someone more experienced would point me that.

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