swift234

Align declarations permissible within protocol bodies with the language reference.

Namely, nested structs, class, enum and other protocol declarations within protocols should raise a parse error, but not crash the type checker.
Also, protocol methods may not have bodies.

The fact that we weren't catching these issues resulted in a number of crasher bugs:
rdar://problem/15811382
rdar://problem/16290458
rdar://problem/16299208
rdar://problem/16316944

Swift SVN r15093