[clang-tidy] extend bugprone-signed-char-misuse check.

Summary:
Cover a new use case when using a 'signed char' as an integer
might lead to issue with non-ASCII characters. Comparing
a 'signed char' with an 'unsigned char' using equality / unequality
operator produces an unexpected result for non-ASCII characters.

Reviewers: aaron.ballman, alexfh, hokein, njames93

Reviewed By: njames93

Subscribers: xazax.hun, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D75749