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

[Sergei Barannikov via llvm-commits]

s-barannikov wrote:

> @abhina-sree @hubert-reinterpretcast @s-barannikov
> 
> Here is a suggestion: lets try to reach completion on this PR and land it, then go through the 2-3 follow up that are open, and then (and only then) we can decide to land or not the driver changes (and documentation)
> 
> The logic is that until there is a driver option, as long as the changes do not cause regressions we can land part of the feature and iterate piece-meal. Hopefully that remove the pressure of a fast-approaching branch date.
> 
> Sounds good?

SGTM

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