SourceKit document structure request used to crash if if there's inititalizer
with single named parameter with `@IBAction` attribute.
This used to happen because
ConstructorDecl::isObjCZeroParameterWithLongSelector() requires
interface type of the parameter but the constructor doesn't have it at
this stage.
This change fixes that by not vending ObjC name for
constructor or destructors.
rdar://problem/47426948
