Autodetected compilers don't get redetected when written settings are wrong

For example when running Qt Creator 4.2, autodetected compilers get "darwin" as ABI on macOS.

Qt Creator 4.1 doesn't know "darwin", since it was "macos" before 4.2, and sets the abi to "unknown", breaking all compilers, and kits.

Unfortunatly that "unknown" is written to the settings, so Qt Creator 4.2 happily reads that as new abis for the compilers and is broken too.

Instead, when broken settings for autodetected compilers are found, it should redetect the settings for it.