[clang] Thread Safety Analysis: Fix crash with wide string literals (PR #180349)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

This should have a release note so users know about the fix.

One thing I'm concerned by is that we have `compareString` functions which I don't think will expect this because those bytes may not be comparable. e.g., we do this:
```
  case ValueType::BT_String:
    return Cmp.compareStrings(as<StringRef>().value(),
                              E->as<StringRef>().value());
```
and is that actually going to work when we don't have a `StringRef` but instead have a bucket of bytes?

https://github.com/llvm/llvm-project/pull/180349