There was a fair amount of code duplication in handling the various
places where @objc could either be explictly specified or be inferred;
centralize these in a new shouldMarkAsObjC() function. NFC
Swift SVN r28125
Introduce basic validation for throwing @objc initializers, e.g., a
failable @objc initializer cannot also be throwing. However,
Objective-C selector computation is broken.
Swift SVN r27292
For any @objc method, attach either the foreign error convention we inherit
from an overridden method (if available) or the foreign error
convention we computed as part of @objc validation.
Extend the AST dumper to dump the foreign error convention so that we
can test this.
Swift SVN r27267
