[clang] [clang][c++20] Fix code coverage mapping crash with generalized NTTPs (PR #85837)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic approved this pull request.

LGTM

(In the future, please leave a note on the pull request when you push an update; as far as 
I can tell, GitHub doesn't generate a notification when you force-push to the branch.)

https://github.com/llvm/llvm-project/pull/85837