swift234

Introduce a protocol conformance registry for nominal types.

(Note that this registry isn't fully enabled yet; it's built so that
we can test it, but has not yet taken over the primary task of
managing conformances from the existing system).

The conformance registry tracks all of the protocols to which a
particular nominal type conforms, including those for which
conformance was explicitly specified, implied by other explicit
conformances, inherited from a superclass, or synthesized by the
implementation.

The conformance registry is a lazily-built data structure designed for
multi-file support (which has been a problematic area for protocol
conformances). It allows one to query for the conformances of a type
to a particular protocol, enumerate all protocols to which a type
conforms, and enumerate all of the conformances that are associated
with a particular declaration context (important to eliminate
duplicated witness tables).

The conformance registry diagnoses conflicts and ambiguities among
different conformances of the same type to the same protocol. There
are three common cases where we'll see a diagnostic:

1) Redundant explicit conformance of a type to a protocol:

    protocol P { }
    struct X : P {  }
    extension X : P { } // error: redundant explicit conformance

2) Explicit conformance to a protocol that collides with an inherited
  conformance:

    protocol P { }
    class Super : P { }
    class Sub : Super, P { } // error: redundant explicit conformance

3) Ambiguous placement of an implied conformance:

    protocol P1 { }
    protocol P2 : P1 { }
    protocol P3 : P1 { }

    struct Y { }
    extension Y : P2 { }
    extension Y : P3 { } // error: ambiguous implied conformance to 'P1'

  This happens when two different explicit conformances (here, P2 and
  P3) placed on different declarations (e.g., two extensions, or the
  original definition and other extension) both imply the same
  conformance (P1), and neither of the explicit conformances imply
  each other. We require the user to explicitly specify the ambiguous
  conformance to break the ambiguity and associate the witness table
  with a specific context.

Swift SVN r26067