[Reg 4.11->4.12] Kits: Automatic selection of auto-detected compiler not working anymore

The WebAssembly plugin auto-detects a compiler with a distinctive ABI. If Qt Creator is launched once with the plugin disabled, the Kit is invalid, because neither compiler nor device are set. That's expected, and we live with that since the WebAssembly Plugin is experimental.

But if Qt Creator is then restarted with the plugin enabled, the kit remains invalid due to non-set compiler. That is a regression in 4.12-beta1, it used to work in 4.11. In 4.11, the Kit would be auto-repaired, because the right compiler gets auto-magically set.

If I start Qt Creator with the WebAssembly plugin on, the first time after installing the Qt versions via SDK, everything still works fine.