Bump version for 2.12.x

For this step, we still depend on _2.11 libraries (partest, xml,
parser-combinators).

After we release the first milestone of 2.12, we will update
versions.property to bootstrap to 2.12 dependencies.
Disable MiMa on the 2.12.x branch
Merge pull request #3715 from retronym/topic/bump

Bump version for 2.12.x
Merge remote-tracking branch 'origin/2.11.x' into merge/2.11.x-to-2.12.x
Make PartialFunction.OrElse extend AbstractPartialFunction.

This shaves a few kB off of the generated class file.
Merge remote-tracking branch 'origin/2.11.x' into merge/2.11.x-to-2.12.x
Merge pull request #3731 from retronym/merge/2.11.x-to-2.12.x

Merge 2.11.x to 2.12.x
Merge pull request #3740 from tvierling/trait-instantiation

Make PartialFunction.OrElse extend AbstractPartialFunction.
SI-8599 Improve type variable inference in nested matches

Refine the upper and lower bounds independentally, rather than
bailing out if `!(lo1 <:< hi1)`.

Before, the enclosed test case traced out as follows:

```
inferTypedPattern
-----------------------------------
pattpt   = RefinementTest.Child[c]
pt       = RefinementTest.Parent[a]
ptparams = [type a]
ptvars   = [=?(?c, ?c)]
ptvars foreach instantiateTypeVar

  instantiateTypeVar
  -----------------------------------
  tvar  = =?(?c, ?c)
  tb0   =  >: A <: A
  instBounds(tvar)

    instBounds
    -----------------------------------
    loBounds   = [(c, c)]
    hiBounds   = [(c, c)]
    tparam     = type a
    val TypeBounds(lo, hi) = tparam.info.bounds
    lo         = A
    hi         = A
    lub(A :: (c, c) :: Nil) = Any
    glb(A :: (c, c) :: Nil) = A with (c, c)
    result = >: Any <: A with (c, c)

  lo1   = Any
  hi1   = A with (c, c)
  if (! (lo1 <:< hi1) )
   log(s"inconsistent bounds: discarding TypeBounds($lo1, $hi1)")
```

So because we haven't yet refined the info of `a`, it still appears
distinct from `(c, c)`, and the LUB is `Any`. Based on this, we
don't proceed to refine the type.

This commit changes the process to individually retain or discard
the low and high bounds, which is enough to tie the knot here.