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

[Alexey Gerenkov via cfe-commits]

gerekon 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.

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