[clang] [WIP][clang]: Implement a conditional lifetimebound_if builtin. (PR #125520)

[Haojian Wu via cfe-commits]

hokein wrote:

> A moved from object could be reinitalized:
> 
> ```
> void test() {  
>     std::vector<std::string_view> abc;  
>     std::string b;  
>     add(abc, std::move(b));  
>     b = std::string(); // now b can be used again. 
> }
> ```
> 
> That being said, maybe this is rare enough that we could have an opt-in warning. But we definitely cannot have something that is on by default.

I think we are primarily concerned with pointers or references to a moved-from object. When the moved-from object is reinitialized, using a pointer to it can be very tricky and is likely undefined behavior, https://godbolt.org/z/Wh5vKe8z6

https://github.com/llvm/llvm-project/pull/125520