[clang] [llvm] [NFC][analyzer] Extract bounds checking library (PR #202372)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

@Xazax-hun One quick question: What is your preferred name for the following helper type?
```c++
struct /*currently Messages*/ {
  std::string Short;
  std::string Full;
};
```
This is used as the return type of methods of the check result, the returned strings will be passed to the  `shortDesc` and `desc` parameters of the constructor of `PathSensitiveBugReport`. (Currently this returns complete messages, but in the future I'll probably generalize it to only return a part of the message -- which would be concatenated with a checker-dependent string that describes the access operation.)

I'm not using a `std::pair` because the names of the data members clarify which string is which.

https://github.com/llvm/llvm-project/pull/202372