Sema: Rewrite witness method calls as ApplyExpr + DeclRefExpr

Special-casing these as MemberRefExprs created an asymmetry
where unbound archetype instance methods (<T : P> T.f) could
not be represented. Treating class and protocol methods
uniformly also eliminates a handful of special cases around
MemberRefExpr.

SILGen's RValue and call emission peepholes now have to know
about DeclRefExprs that point to protocol methods.

Finally, generalize the diagnostic for partially applied
mutating methods to any partially applied function with an
inout parameter, since this is not supported.

Fixes <rdar://problem/20564672>.

Swift SVN r29298

lib/Sema/ConstraintSystem.cpp

@@ -1010,14 +1010,7 @@ void ConstraintSystem::openGeneric(
 /// declared.
 static void addSelfConstraint(ConstraintSystem &cs, Type objectTy, Type selfTy,
                               ConstraintLocatorBuilder locator){
-  // When referencing a protocol member, we need the object type to be usable
-  // as the Self type of the protocol, which covers anything that conforms to
-  // the protocol as well as existentials that include that protocol.
-  if (selfTy->is<ProtocolType>()) {
-    cs.addConstraint(ConstraintKind::SelfObjectOfProtocol, objectTy, selfTy,
-                     cs.getConstraintLocator(locator));
-    return;
-  }
+  assert(!selfTy->is<ProtocolType>());
 
   // Otherwise, use a subtype constraint for classes to cope with inheritance.
   if (selfTy->getClassOrBoundGenericClass()) {