[clang] Fix clang crash when printing highlighted code in diagnostic (after #66514) (PR #80442)

[Aaron Ballman via cfe-commits]

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

LGTM to get this unblocked, but I would love it if we could add a test case post-commit.

https://github.com/llvm/llvm-project/pull/80442