swift234

Introduce TypeBase::isExistentialType(), to determine whether a given
type is either a protocol type or a protocol composition type. The
long form of this query returns the minimal set of protocol
declarations required by that existential type.

Use the new isExistentialType() everywhere that we previously checked
just for ProtocolType, implementing the appropriate rules. Among other
things, this includes:
  - Type coercion
  - Subtyping relationship
  - Checking of explicit protocol conformance
  - Member name lookup

Note the FIXME for IR generation; we need to decide how we want to
encode the witnesses for the different protocols.

This is most of <rdar://problem/11548207>.


Swift SVN r2086