Factor-out the `OverlapMode` from `OverlapFlag`.

This seems a bit cleaner conceptually because the overlap mode and running
in safety mode are quite orthogonal.

More pragmatically, it also makes it possible to use `OverlapMode` to let
programmers pick the overlap mode for individual instances.
Overlapable pragmas for individual instances (#9242)

Programmers may provide a pragma immediately after the `instance` keyword
to control the overlap/incoherence behavior for individual instances.
For example:

    instance {-# OVERLAP #-} C a where ...

I chose this notation, rather than the other two outlined in the ticket
for these reasons:

   1. Having the pragma after the type looks odd, I think.
   2. Having the pragma after there `where` does not work for
       stand-alone derived instances

I have implemented 3 pragams:

   1. NO_OVERLAP
   2. OVERLAP
   3. INCOHERENT

These correspond directly to the internal modes currently supported by
GHC.  If a pragma is specified, it will be used no matter what flags are
turned on.   For example, putting `NO_OVERLAP` on an instance will mark
it as non-overlapping, even if `OVERLAPPIN_INSTANCES` is turned on for the
module.
Eliminate `Unify.validKindShape` (#9242)