swift234

[Sema -> AST] Refactor "is representable in Objective-C?" checking.

Migrate the check for whether a given type is representable in
Objective-C, which is currently used to verify when @objc can be
inferred or verify that an explicitly-written @objc is well-formed,
from Sema into a set of queries on the Type within the AST library, so
it can be used in other parts of the compiler.

As part of this refactoring, clean up and improve a number of aspects
of this code:

* Unify the "trivially representable" and "representable" code paths
  into a single code path that covers these cases. Clarify the
  different levels of "representable" we have in both the code and
  in comments.

* Distinguish between representation in C vs. representation in
  Objective-C. While we aren't using this now, I'm anticipating it
  being useful to allow exporting C interfaces via @_cdecl (or
  similar).

* Eliminate the special cases for bridging String/Array/Dictionary/Set
  with their Foundation counterparts; we now consult
  _ObjectiveCBridgeable conformances exclusively to get this
  information.

* Cache foreign-representation information on the ASTContext in a
  manner that will let us more easily get the right answer across
  different contexts while providing more sharing than the TypeChecker
  version.

Annoyingly, this only seemed to fix a small class of error where we
were permitting Unsafe(Mutable)Pointer<T> to be representable in
Objective-C when T was representable but not trivially representable,
e.g., T=String or T=AnyObject.Type.