Introduce scaffolding for -Xsource:2.12

Any changes to implicit search or type inference that we want to
ship in the 2.11.x series should be guarded by this flag. This is
philosophically equivalent to `-Xfuture`, and indeed we should
try to merge these flags.

Such changes can only be added to 2.11.x if we can reason that they
will interop with separately compiled code that doesn't use
the flag. An example of a change that would not qualify: fixes that
modify the erased signatures (e.g. AsSeenFrom or ErasureMap); caller
and callee must agree on these matters.
SI-8597 Predicate change in inferTypedPattern on -Xsource:2.12

I haven't found a test that breaks with the change in place, but
lets play it safe and leave the old behaviour in place for 2.11.x.