[llvm] fix: add encoding for Python 3.14 compatibility in lit test file (PR #178261)

[Igor Luppi via llvm-commits]

igorluppi wrote:

pops up this:
```
Setting up llvm-21-tools (1:21.1.8-1+b1) ...
  File "/usr/lib/llvm-21/build/utils/lit/tests/shtest-encoding.py", line 3
    # Here is a string that cannot be decoded in line mode: �.
                                                            ^
SyntaxError: Non-UTF-8 code starting with '\xc2' on line 3, but no encoding
```

https://github.com/llvm/llvm-project/pull/178261