Sema: Diagnose unbound method references on 'super.'

This is something I noticed by inspection while working on
<https://bugs.swift.org/browse/SR-75>.

Inside a static method, 'self' is a metatype value, so
'self.instanceMethod' produces an unbound reference of type
(Self) -> (Args...) -> Results.

You might guess that 'super.instanceMethod' can similarly
be used to produce an unbound method reference that calls
the superclass method given any 'self' value, but unfortunately
it doesn't work.

Instead, 'super.instanceMethod' would produce the same
result as 'self.instanceMethod'. Maybe we can implement this
later, but for now, let's just diagnose the problem.

Note that partially-applied method references with 'super.'
-- namely, 'self.staticMethod' inside a static context, or
'self.instanceMethod' inside an instance context, continue
to work as before.

They have the type (Args...) -> Result; since the self value
has already been applied we don't hit the representational
issue.

lib/Sema/CSDiagnostics.cpp

@@ -3756,6 +3756,8 @@ bool PartialApplicationFailure::diagnoseAsError() {
           anchor, ConstraintLocator::ConstructorMember))) {
     kind = anchor->getBase()->isSuperExpr() ? RefKind::SuperInit
                                             : RefKind::SelfInit;
+  } else if (anchor->getBase()->isSuperExpr()) {
+    kind = RefKind::SuperMethod;
   }
 
   auto diagnostic = CompatibilityWarning