[clang-tidy] misc-string-integer-assignment: fix false positive

Summary:
using CodePoint = uint32_t;
CodePoint cp;
basic_string<CodePoint> s;
s += cp;

See PR27723.

Reviewers: xazax.hun, alexfh

Subscribers: rnkovacs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D58606

llvm-svn: 355076