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

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

@shafik, @erichkeane @AaronBallman @tbaederr @zwuis 
I believe all feedback has been addressed.  Could you give it another look when you are free?

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