[clang] [clang-tools-extra] [flang] Fix OOM in FormatDiagnostic (2nd attempt) (PR #108866)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Thank you! I think this fixes everything caught by post-commit, so let's re-land.

https://github.com/llvm/llvm-project/pull/108866