Jordan Rose a6dd630ca3 Eliminate Builtin.UnknownObject as an AST type (#27378) ...

This removes it from the AST and largely replaces it with AnyObject
at the SIL and IRGen layers. Some notes:

- Reflection still uses the notion of "unknown object" to mean an
  object with unknown refcounting. There's no real reason to make
  this different from AnyObject (an existential containing a
  single object with unknown refcounting), but this way nothing
  changes for clients of Reflection, and it's consistent with how
  native objects are represented.

- The value witness table and reflection descriptor for AnyObject
  use the mangling "BO" instead of "yXl".

- The demangler and remangler continue to support "BO" because it's
  still in use as a type encoding, even if it's not an AST-level
  Type anymore.

- Type-based alias analysis for Builtin.UnknownObject was incorrect,
  so it's a good thing we weren't using it.

- Same with enum layout. (This one assumed UnknownObject never
  referred to an Objective-C tagged pointer. That certainly wasn't how
  we were using it!)

2019-09-26 17:48:04 -07:00

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