[clang] [llvm] Enable fexec-charset option (PR #138895)

[Felipe de Azevedo Piovezan via cfe-commits]

felipepiovezan wrote:

[This is making the standalone builds of LLDB fail with:
](https://ci.swift.org/view/all/job/llvm.org/job/lldb-cmake-standalone/3347/console)
```
[2026-07-02T20:25:08.456Z] /Users/ec2-user/jenkins/workspace/llvm.org/lldb-cmake-standalone/clang-install/include/llvm/Support/TextEncoding.h:20:10: fatal error: 'llvm/Config/config.h' file not found
  [2026-07-02T20:25:08.456Z]    20 | #include "llvm/Config/config.h"
```



https://github.com/llvm/llvm-project/pull/138895