[clang] [Xtensa] Default to unsigned char (PR #115967)

[Alexey Gerenkov via cfe-commits]

gerekon wrote:

> Looking at #118008 it sounds like we don't yet have complete codegen support for XTensa in Clang, so it sounds like this is safe without a flag.

Yes, it is not upstreamed yet. We are working on it. But after rebasing on the LLVM 20 we might get this fix in our repo, so that will influence our Clang toolchain and Rust, Swift ones. So I asked @MabezDev if he sees any potential problems for this.

https://github.com/llvm/llvm-project/pull/115967