SR-12486: T.self is Any.Protocol is broken

This turned out to be fallout from #27572
which was in turn motivated by our confusing metatype syntax when generic variables are bound to protocols.

In particular, the earlier PR was an attempt to make the expression
`x is T.Type` (where `T` is a generic type variable bound to a protocol `P`)
behave the same as
`x is P.Type` (where `P` is a protocol).
Unfortunately, the generic `T.Type` actually binds to `P.Protocol` in this case (not `P.Type`), so the original motivation was flawed, and as it happens, `x is T.Type` already behaved the same as `x is P.Protocol` in this situation.

This PR reverts that earlier change and beefs up some of the tests around these behaviors.

Resolves SR-12486

Resolves rdar://62201613

Reverts PR#27572

Author: tbkka

stdlib/public/runtime/Casting.cpp

@@ -1185,13 +1185,6 @@ swift_dynamicCastMetatypeImpl(const Metadata *sourceType,
     }
     break;
 
-  case MetadataKind::Existential: {
-    auto targetTypeAsExistential = static_cast<const ExistentialTypeMetadata *>(targetType);
-    if (_conformsToProtocols(nullptr, sourceType, targetTypeAsExistential, nullptr))
-      return origSourceType;
-    return nullptr;
-  }
-
   default:
     return nullptr;
   }