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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! 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.

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

The changes are mostly looking good to me, but more sets of eyes on the changes would be appreciated.

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