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

[Hubert Tong via llvm-commits]

================
@@ -217,8 +217,14 @@
 // RUN: not %clang -### -S -finput-charset=iso-8859-1 -o /dev/null %s 2>&1 | FileCheck -check-prefix=CHECK-INVALID-CHARSET %s
 // CHECK-INVALID-CHARSET: error: invalid value 'iso-8859-1' in '-finput-charset=iso-8859-1'
----------------
hubert-reinterpretcast wrote:

```suggestion
// RUN: not %clang -### -S -finput-charset=iso-8859-1 -o /dev/null %s 2>&1 | FileCheck -check-prefix=CHECK-INVALID-INPUT-CHARSET %s
// CHECK-INVALID-INPUT-CHARSET: error: invalid value 'iso-8859-1' in '-finput-charset=iso-8859-1'
```

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