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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> but this is going to need a lot more tests

For preprocessor string concatenation and escape sequences within strings, the shift-sequence behaviour for stateful encodings would be of interest.

Additionally of interest is the treatment of bytes matching the encoding of `%`, etc. in a non-initial shift state within a `printf`, etc. format string.

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