Use RuntimeHelpers.GetHashCode in IndexObject (#7678)

## Summary of changes

This changes to use a means to get the hash code of an object to handle
`null`, `object.GetHashCode` and a fallback to use RuntimeHelpers
`GetHashCode` for when `object.GetHashCode` fails.

## Reason for change

My guess is that `GetHashCode` is overridden for the given object and we
probably should avoid that?

Spotted the following in Error Tracking:

```
Error : PropagationModuleImpl.PropagateWholeResultWhenInputTainted exception
System.NullReferenceException
   at REDACTED
   at Datadog.Trace.Iast.DefaultTaintedMap.IndexObject(Object objectStored)
   at Datadog.Trace.Iast.DefaultTaintedMap.Get(Object objectToFind)
   at Datadog.Trace.Iast.Propagation.PropagationModuleImpl.PropagateWholeResultWhenInputTainted(String result, Object input1, Object input2, Object input3)
```

## Implementation details

Swapped from one to the other.

## Test coverage

🤷 Didn't try to reproduce

## Other details
<!-- Fixes #{issue} -->


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Author: bouwkast

tracer/src/Datadog.Trace/Iast/DefaultTaintedMap.cs

@@ -228,7 +228,7 @@ public void Clear()
 
     private int IndexObject(object objectStored)
     {
-        return Index(PositiveHashCode(objectStored.GetHashCode()));
+        return Index(PositiveHashCode(IastUtils.IdentityHashCode(objectStored)));
     }
 
     private int PositiveHashCode(int hash)

tracer/src/Datadog.Trace/Iast/IastUtils.cs

@@ -71,7 +71,20 @@ private static int GetHash<T>(T element)
 
     public static int IdentityHashCode(object item)
     {
-        return (item?.GetHashCode() ?? 0);
+        if (item == null)
+        {
+            return 0;
+        }
+
+        try
+        {
+            return item.GetHashCode();
+        }
+        catch
+        {
+            // Seen this that it appears some customer objects have overrode GetHashCode and throw
+            return System.Runtime.CompilerServices.RuntimeHelpers.GetHashCode(item);
+        }
     }
 
     public static Range[] GetRangesForString(string stringValue, Source source)