swift234

Fix rdar://21311590 QoI: Inconsistent diagnostics when no constructor is available

In the case of an invalid override of an init, mark the override invalid, not the
init.  This way, code that uses the init can be parsed and analyzed correctly and
bogus downstream errors are reduced.

Before:

t.swift:2:12: error: initializer does not override a designated initializer from its superclass
  override init() {}
  ~~~~~~~~ ^
t.swift:5:1: error: cannot invoke initializer for type 'C' with no arguments
C()
^
t.swift:6:1: error: 'B' cannot be constructed because it has no accessible initializers
B()
^

After:

t.swift:2:12: error: initializer does not override a designated initializer from its superclass
  override init() {}
  ~~~~~~~~ ^