[clang-tools-extra] [clang-tidy] Teach `performance-faster-string-find` about `operator+=` (PR #182697)

[Victor Chernyakin via cfe-commits]

localspook wrote:

Based on the check name, I agree, that's the more fitting place for it. But that would spread what is conceptually one optimization (prefer single-character overloads) out over multiple checks (we would have to, for example, factor `makeCharacterLiteral` in this file out into a shared function). In my opinion, that's a worse tradeoff. What do others think?

(This could have been avoided if `performance-faster-string-find` was called something like `performance-inefficient-string-operation`...)

https://github.com/llvm/llvm-project/pull/182697