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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> I have an idea how to make this progr_e_ss. How about extracting the option handling in a separate PR? I guess this is the less controversial change and could be landed independently. That is, add the `-fexec-charset` option, process it and pass from the driver to the compiler, which will define `__clang_literal_encoding`, and that's all. This will also ensure that the driver/preprocessor changes have good test coverage.

I agree that splitting will help the review; however, I don't think we should land the driver part _first_. It should probably be added last.

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