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

[Baranov Victor via cfe-commits]

vbvictor wrote:

> IMO this belongs into `performance-inefficient-string-concatenation`.

+1 on this if it's not hard to do inside that check

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