[clang] [-Wunterminated-string-initialization] Handle C string literals ending with explicit '\0' (PR #143487)

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

Thank you all for valuable feedbacks.  I have improved the PR by
- supporting different character encodings
- adding more test cases
- updating the release notes

https://github.com/llvm/llvm-project/pull/143487