[clang] [analyzer][NFC] Take BugReport descriptions as Twine instead of StringRef (PR #205527)

[Benedek Kaibas via cfe-commits]

benedekaibas wrote:

This is great, thank you! I am also wondering if we want to add `getString()` method to `SVal`. If we would done that then analyzers using `SVal` could make a use of `Twine` by allowing a variable that has an `SVal` type to be printed by using `Twine`.

As an example:

```cpp
llvm::SmallString<128> Str;
 llvm::raw_svector_ostream OS(Str);
OS << " Origin " << ArgSVal << " bound to " << Region;
auto BR = std::make_unique<PathSensitiveBugReport>(BugMsg, OS.str(), N);
C.emitReport(std::move(BR));
```

Could be simply written as:

```cpp
  auto BR = std::make_unique<PathSensitiveBugReport>(BugMsg, llvm::Twine(" Origin ") + ArgSVal.getString() + " bound to " +
      Region->getString(), N);
```

AFAIK this change would require to add `getString()` method to `SVal` similar to how `MemRegion::getString()` works. If you agree, I'd love to work on this feature.

https://github.com/llvm/llvm-project/pull/205527