[reflection] Generalize the implementation of getInstanceTypeInfo()

This is being indirectly tested by the LLDB testsuite.

Author: adrian-prantl

include/swift/RemoteInspection/ReflectionContext.h

@@ -1038,10 +1038,12 @@ class ReflectionContext
       // Generic SIL @box type - there is always an instantiated metadata
       // pointer for the boxed type.
       if (auto Meta = readMetadata(*MetadataAddress)) {
-        auto GenericHeapMeta =
-          cast<TargetGenericBoxHeapMetadata<Runtime>>(Meta.getLocalBuffer());
-        return getMetadataTypeInfo(GenericHeapMeta->BoxedType,
-                                   ExternalTypeInfo);
+        if (auto *GenericHeapMeta = cast<TargetGenericBoxHeapMetadata<Runtime>>(
+                Meta.getLocalBuffer())) {
+          auto MetadataAddress = GenericHeapMeta->BoxedType;
+          auto TR = readTypeFromMetadata(MetadataAddress);
+          return getTypeInfo(TR, ExternalTypeInfo);
+        }
       }
       return nullptr;
     }