[clang-tidy]: Add cert-str34-c alias for bugprone-signed-char-misuse.

Summary:
Added `DiagnoseSignedUnsignedCharComparisons` option to
filter out unrelated use cases. The SEI cert catches explicit
integer casts (two use cases), while in the case of
`signed char` \ `unsigned char` comparison, we have implicit
conversions.

Reviewers: aaron.ballman

Reviewed By: aaron.ballman

Subscribers: xazax.hun, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79334