Attribute serialization doesn't track source indices for params

[jcouv]

The logic that prepares values for attribute serialization, in the proper order, keeps track of where the value came from in source.
To do so, it uses source indices.
When a default value is used, there is no source indice, so -1 is used.
Currently, for params parameters, -1 is also used. But we should be able to track something better for those too (ie. not -1).

See GetRewrittenAttributeConstructorArguments method in Binder_Attributes.cs.

Those indices are used for reporting errors on well-known attributes (which currently are simple and don't involve params), so this is not blocking or user-visible yet.

Relates to #22231 (review)