[clang] [clang] Stop simplifyConstraint at embedded NUL (PR #196223)

[Jeremy Morse via cfe-commits]

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

LGTM on the basis that not crashing is an improvement, as reproduced in #173900. There's a risk that there are other paths which will now see embedded nulls, which aren't easily discoverable -- what does gcc do for this input? It might be better to add an early diagnostic/error rejecting such embedded nulls, rather than try to handle nulls once they're inside clang.

(CC'd Eli as he was involved in the original patch #154905).

https://github.com/llvm/llvm-project/pull/196223