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

[Mark Cianciosa via cfe-commits]

cianciosa wrote:

This is causing a compile error on macOS.

```
/LLVM/clang/include/clang/Lex/TextEncoding.h:18:7: error: definition of type 'TextEncoding' conflicts with typedef of the same name
   18 | class TextEncoding {
      |       ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX26.4.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/TextCommon.h:586:41: note: 
      'TextEncoding' declared here
  586 | typedef UInt32                          TextEncoding;
      |                                         ^
```

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