swift234

dramatically improve error recovery when "typealias" keyword is used in a
class or struct conforming to a protocol.  Now we produce a single error
with a fixit hint (rewriting to typealias).  Before we produced:

t.swift:7:3: error: associated types can only be defined in a protocol; define a type or introduce a 'typealias' to satisfy an associated type requirement
  associatedtype T = Int
  ^
t.swift:7:17: error: consecutive declarations on a line must be separated by ';'
  associatedtype T = Int
                ^
                ;
t.swift:7:18: error: expected declaration
  associatedtype T = Int
                 ^
t.swift:6:7: error: type 'C' does not conform to protocol 'P'
class C : P {
      ^
t.swift:3:18: note: protocol requires nested type 'T'
  associatedtype T
                 ^