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

[Vakhurin Sergei via cfe-commits]

igelbox wrote:

Well, the more it waits the bigger a probability it will have logical conflicts after merging into `main`.

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