[clang-tidy] bugprone-string-integer-assignment: Reduce false positives.

Summary: Detect a few expressions as likely character expressions, see PR27723.

Reviewers: xazax.hun, alexfh

Subscribers: rnkovacs, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58609

llvm-svn: 355089