GHCi recompilation avoidance UI

[Evan Laforge]

I updated the trac ticket, but here's a copy-paste:

I still don't feel like 1 is necessary, I'd rather flags cause other
flags to be ignored with a warning, rather than turn on other flags.
But that's just a vague preference, with no strong evidence for it.
Maybe it could emit a warning if you didn't put -fobject-code in
explicitly, e.g. "-O implies -fobject-code, adding that flag." So as
long as we accept 1, then 2 and 3 follow naturally. Given that, I
support this UI.

Thanks for looking into it!