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

[Alexander Richardson via cfe-commits]

arichardson wrote:

> Hi @arichardson.
> 
> Thanks for this change. Acc to the doc which I have `The char type is unsigned by default for Xtensa processors.` Our GCC Xtensa toolchain also interprets char type as unsigned by default.

Thanks, do you believe we need a backwards compatibility flag for the ABI to be consistent with older clang or is this safe to land?

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