[clang] [CSKY] Default to unsigned char (PR #115961)

[Alexander Richardson via cfe-commits]

arichardson wrote:

> LGTM. But I think commit msg should be refined(this PR is not for upstreaming?)

Thanks for reviewing!
Sorry what do you mean by commit message needs to be refined? I created this using the spr tool and when merging the description of the PR will be used. Is there something wrong with the description?

https://github.com/llvm/llvm-project/pull/115961