[clang] [llvm] Use %errc to get text for system errors (PR #109852)

[David Spickett via cfe-commits]

DavidSpickett wrote:

CI failure is:
```
_bk;t=1727216861348Timed Out Tests (1):
_bk;t=1727216861348  lldb-api :: functionalities/fork/concurrent_vfork/TestConcurrentVFork.py
```

Ignore that it's due to https://github.com/llvm/llvm-project/issues/104609.

https://github.com/llvm/llvm-project/pull/109852