[clang-tools-extra] [clang-tidy] Fix false positives in `bugprone-signed-char-misuse` (PR #149790)

[Björn Svensson via cfe-commits]

bjosv wrote:

> what about enum with unsigned underlying type. it looks not covered by tests

I added a test for enums with unsigned underlying type.
I also added additional cases that should be ignored by the check, similar to what exists for C++.

https://github.com/llvm/llvm-project/pull/149790