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

[Erich Keane via cfe-commits]

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

This seems fine to me, but please give @efriedma-quic a few days to do a final pass through.

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