AST: Introduce primitive AnyObject type

Add a 'hasExplicitAnyObject()' bit to ProtocolCompositionType
to represent canonical composition types containing '& AnyObject'.

Serialize this bit and take it into account when building
ExistentialLayouts.

Rename ProtocolCompositionType::getProtocols() to getMembers()
since it can contain classes now, and update a few usages that
need further attention with FIXMEs or asserts.

For now, nothing actually constructs these types, and they will
trigger arounds asserts. Upcoming patches will introduce support
for this.

lib/AST/GenericSignatureBuilder.cpp

@@ -2199,13 +2199,16 @@ static ConstraintResult visitInherited(
   // Local function that (recursively) adds inherited types.
   ConstraintResult result = ConstraintResult::Resolved;
   std::function<void(Type, const TypeRepr *)> visitInherited;
+
+  // FIXME: Should this whole thing use getExistentialLayout() instead?
+
   visitInherited = [&](Type inheritedType, const TypeRepr *typeRepr) {
     // Decompose protocol compositions.
     auto composition = dyn_cast_or_null<CompositionTypeRepr>(typeRepr);
     if (auto compositionType
           = inheritedType->getAs<ProtocolCompositionType>()) {
       unsigned index = 0;
-      for (auto protoType : compositionType->getProtocols()) {
+      for (auto protoType : compositionType->getMembers()) {
         if (composition && index < composition->getTypes().size())
           visitInherited(protoType, composition->getTypes()[index]);
         else