Change INDEX property default value to -2.

[ml86]

Previously INDEX property default value was -1 now it is -2.
We do this to avoid representation collisions if frontends happen to not
set this property. See code comment for further details.

[SuperUserDone]

Might need to have a look here: https://github.com/ShiftLeftSecurity/codescience/blob/master/semanticcpg-ext/src/main/scala/io/shiftleft/semanticcpgext/passes/varargparamindex/VarArgParameterPass.scala

[ml86]

@SuperUserDone The VarArgParameterPass is fine. The -1 encoding of "method has no vararg parameter is independent of the INDEX default value.