[clang-tidy] misc-string-integer-assignment: ignore toupper/tolower

Summary: Tis represents ~20% of false positives. See PR27723.

Reviewers: xazax.hun, alexfh

Subscribers: rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58604

llvm-svn: 354780