Rollup merge of #138282 - beetrees:repr128-not-ffi-safe, r=oli-obk

Add `#[repr(u128)]`/`#[repr(i128)]` enums to `improper_ctypes_definitions`

This makes them warn whenever a plain `u128`/`i128` would. If the lang team decides to merge #137306 then this can be reverted.

Tracking issue: #56071

Author: matthiaskrgr