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

[Sergei Barannikov via cfe-commits]

s-barannikov 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.

Is that because you don't want to leave the option in "half-working" state? I thought that would be okay, I don't expect the users to rush and use it right away. I'm not opposed to doing this as the last step, but is this reqally required?

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