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

[Sergei Barannikov via cfe-commits]

s-barannikov wrote:

ping @cor3ntin, could you do another round?
This PR has significantly shrunk in size making it easier for review
I feel like my review isn't very useful and feedback from a clang mantainer could move this forward, it has been half a year already

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