[clang] [clang][Python] Use fstrings instead of string concatenations (PR #173861)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

That said, I agree that here f-string are an improvement, because it's at the end of the string, I guess
```python
            msg = f"Error reparsing translation unit. Error code: {result}"
```

https://github.com/llvm/llvm-project/pull/173861