[clang-tidy] Fix more false positives for bugprone-string-integer-assignment

Summary:
And add various tests gleaned for our codebase.

See PR27723.

Reviewers: JonasToth, alexfh, xazax.hun

Subscribers: rnkovacs, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59360

llvm-svn: 356871