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

[Alexander Richardson via cfe-commits]

arichardson wrote:

Looking at https://github.com/llvm/llvm-project/pull/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.

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