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

[Oliver Stöneberg via cfe-commits]

firewave wrote:

> > IMO this belongs into `performance-inefficient-string-concatenation`.
> 
> +1 on this if it's not hard to do inside that check

Please note that check is a bit broken right now though and should not be enabled, so nobody might benefit from this improvement. See #54526, #29206 and #59478.

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