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

[Vakhurin Sergei via cfe-commits]

igelbox wrote:

> Testing Time: 301.48s
Total Discovered Tests: 105926
  Skipped          :     23 (0.02%)
  Unsupported      :   2682 (2.53%)
  Passed           : 103036 (97.27%)
  Expectedly Failed:    185 (0.17%)
1 warning(s) in tests

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