RemoteAST: Fix Error existential introspection on Linux

We don't have tests that look at the instance address
on the Swift side, but lldb is going to use this API
shortly.

Author: slavapestov

include/swift/Remote/MetadataReader.h

@@ -410,11 +410,15 @@ class MetadataReader {
     StoredPointer InstanceAddress =
         InstanceMetadataAddressAddress + 2 * sizeof(StoredPointer);
 
+    // When built with Objective-C interop, the runtime also stores a conformance
+    // to Hashable and the base type introducing the Hashable conformance.
+    if (isObjC)
+      InstanceAddress += 2 * sizeof(StoredPointer);
+
     // Round up to alignment, and we have the start address of the
     // instance payload.
     auto AlignmentMask = VWT->getAlignmentMask();
-    auto Offset = (sizeof(HeapObject) + AlignmentMask) & ~AlignmentMask;
-    InstanceAddress += Offset;
+    InstanceAddress = (InstanceAddress + AlignmentMask) & ~AlignmentMask;
 
     return RemoteExistential(
         RemoteAddress(*InstanceMetadataAddress),