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

[Baranov Victor via cfe-commits]

vbvictor wrote:

> I would personally prefer renaming the check (add the new name as an alias, deprecate the existing one for two releases, etc.). It’ll take longer, but I feel that would get us to a better place in the end.

I'm +1 on the renaming of the check

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