swift234

Add a Builtin.BridgeObject type.

This is a type that has ownership of a reference while allowing access to the
spare bits inside the pointer, but which can also safely hold an ObjC tagged pointer
reference (with no spare bits of course). It additionally blesses one
Foundation-coordinated bit with the meaning of "has swift refcounting" in order
to get a faster short-circuit to native refcounting. It supports the following
builtin operations:

- Builtin.castToBridgeObject<T>(ref: T, bits: Builtin.Word) ->
  Builtin.BridgeObject

  Creates a BridgeObject that contains the bitwise-OR of the bit patterns of
  "ref" and "bits". It is the user's responsibility to ensure "bits" doesn't
  interfere with the reference identity of the resulting value. In other words,
  it is undefined behavior unless:

    castReferenceFromBridgeObject(castToBridgeObject(ref, bits)) === ref

  This means "bits" must be zero if "ref" is a tagged pointer. If "ref" is a real
  object pointer, "bits" must not have any non-spare bits set (unless they're
  already set in the pointer value). The native discriminator bit may only be set
  if the object is Swift-refcounted.

- Builtin.castReferenceFromBridgeObject<T>(bo: Builtin.BridgeObject) -> T

  Extracts the reference from a BridgeObject.

- Builtin.castBitPatternFromBridgeObject(bo: Builtin.BridgeObject) -> Builtin.Word

  Presents the bit pattern of a BridgeObject as a Word.

BridgeObject's bits are set up as follows on the various platforms:

i386, armv7:

  No ObjC tagged pointers
  Swift native refcounting flag bit: 0x0000_0001
  Other available spare bits:        0x0000_0002

x86_64:

  Reserved for ObjC tagged pointers: 0x8000_0000_0000_0001
  Swift native refcounting flag bit: 0x0000_0000_0000_0002
  Other available spare bits:        0x7F00_0000_0000_0004

arm64:

  Reserved for ObjC tagged pointers: 0x8000_0000_0000_0000
  Swift native refcounting flag bit: 0x4000_0000_0000_0000
  Other available spare bits:        0x3F00_0000_0000_0007

TODO: BridgeObject doesn't present any extra inhabitants. It ought to at least provide null as an extra inhabitant for Optional.

Swift SVN r22880