swift234

Sema: Generic classes and subclasses of generic classes now inherit required initializers

Initializers are inherited by synthesizing an implicit decl which
delegates to super.init(). Previously this was only done if the
class and superclass were concrete.

The only thing missing was that we weren't computing an interface
type for the synthesized constructor. There are two steps to this:

- First, we must map the contextual types of the superclass
  initializer's ParamDecls to the subclass generic context.

- Second, we must set the interface type by calling the new
  configureInterfaceType() method, extracted from from
  validateGenericSignature().

Note that configureInterfaceType() now uses the new
AbstractFunctionDecl::hasThrows() flag to set the 'throws' bit on
the function type. Previously, validateGenericFuncSignature()
would look at getThrowsLoc().isValid(), which is not correct for
imported, implicitly-generated or de-serialized decls that 'throw',
because none of those have source location information.

We still don't allow inheriting initializers which have their
own generic parameter list, like 'init<T>(t: T) {...}'. That
requires a little bit more refactoring.

Progress on <rdar://problem/23376955>.