Use ExtensionDecl::getExtendedNominal() to wire up extensions to their
nominal types early in type checking (the bindExtensions()) operation,
rather than going through type validation to do so.
Introduce ExtensionDecl::getExtendedNominal() to provide the nominal
type declaration that the extension declaration extends. Move most
of the existing callers of the callers to getExtendedType() over to
getExtendedNominal(), because they don’t need the full type information.
ExtensionDecl::getExtendedNominal() is itself not very interesting yet,
because it depends on getExtendedType().
During "expansion" of the requirements of a protocol, we check all of
the inherited associated types against definitions within the
protocol. Also look for concrete types within extensions of that
protocol, so we can identify more places where developers have used
typealiases in inheriting protocols to effect a same-type constraint
on an inherited associated type.
Fixes SR-7097 / rdar://problem/38001269.
