swift234

Make better use of Builtin.BridgeObject bits

We used to reserve a specific spare bit to say "this is a native
object."  Now, we're going to say, "if *any* spare bit is set, this is a
native object."  At the cost of having no spare bits to work with in the
non-native case, this allows us to store a number in 1..<4 (actually
0..<4, at some cost in speed for the 0 case) along with any native
object on all platforms.

This half bit advantage is important on 32-bit platforms, we have only
spare 2 bits to work with.

Given that on the 64-bit platforms there are *no* spare bits in the case
where the object is a non-native tagged pointer, we have no guarantee of
being able to store extra information along with an arbitrary non-native
object.  Giving up the ability to store bits for *all* non-native
objects (even non-tagged ones) is therefore not much of a sacrifice.

Fixes <rdar://problem/18920415> More useful spare bits in Builtin.BridgeObject

Swift SVN r23345