Also, if warning about an accessor that comes from a stored property,
point to the property rather than the (implicit, source-location-less)
accessor decl.
Both of these changes are aimed at improving the presentation in Xcode.
rdar://problem/19927828
Swift SVN r25725
Remove the logic that allowed an extension to provide an Objective-C
method that was already declared in the class itself, relying on the
existing Objective-C method redeclaration logic to detect such
conflicts. Fixes rdar://problem/17687082.
Swift SVN r25175
