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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I've reverted; @igelbox, you can create a new PR that addresses the failures found by post-commit CI and I'll review them.

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