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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> > unless we only keep the cc1 option here.
> 
> That is what we should use in most tests. In the driver tests we should only use `-fexec-charset` to verify that it is correctly passed down to the compiler.

@s-barannikov, what you said sounds right.

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