Do not get the absolute value of Map's hash code (#6072)

Fixes #6068, where you could cause a `System.OverFlowException` by using a key and value that leads to calling `abs` on `Int32.MinValue`. This is invalid on .NET. The fix is to not call `abs`, since there's really no reason to make a hash code always positive.

src/fsharp/FSharp.Core/map.fs

@@ -558,7 +558,7 @@ namespace Microsoft.FSharp.Collections
             for (KeyValue(x,y)) in this do
                 res <- combineHash res (hash x)
                 res <- combineHash res (Unchecked.hash y)
-            abs res
+            res
 
         override this.Equals(that) = 
             match that with 
@@ -744,4 +744,4 @@ namespace Microsoft.FSharp.Collections
         let empty<'Key,'Value  when 'Key : comparison> = Map<'Key,'Value>.Empty
 
         [<CompiledName("Count")>]
-        let count (table:Map<_,_>) = table.Count
\ No newline at end of file
+        let count (table:Map<_,_>) = table.Count