swift234

Start allowing extensions of protocol types.

Remove the semantic restrictions that prohibited extensions of
protocol types, and start making some systematic changes so that
protocol extensions start to make sense:
  - Replace a lot of occurrences of isa<ProtocolDecl> and
    dyn_cast<ProtocolDecl> on DeclContexts to use the new
    DeclContext::isProtocolOrProtocolExtensionContext(), where we want
    that behavior to apply equally to protocols and protocol extensions.
  - Eliminate ProtocolDecl::getSelf() in favor of
    DeclContext::getProtocolSelf(), which produces the appropriate
    generic type parameter for the 'Self' of a protocol or protocol
    extension. Update all of the callers of ProtocolDecl::getSelf()
    appropriately.
  - Update extension validation to appropriately form generic
    parameter lists for protocol extensions.
  - Methods in protocol extensions always use the witnesscc calling
  convention.

At this point, we can type check and SILGen very basic definitions of
protocol extensions (without associated types, IRGen crashes, etc.)
with methods that can call protocol requirements, generic free
functions, and other methods within the same protocol extension.

This is identical to r26579; the prior commit addressed the underlying
conformance substitution problem that caused rdar://problem/20320393
and subsequent reversion of r26579.

Swift SVN r26639