Introduce SynPat.ParametersOwner.

[nojaf]

This is a subset, the first step if you will, from #12989.
In 12989 I tried to do too much at the same time and I would like to cut it up into multiple PRs.

This first one refactors SynPat.LongIdent to SynPat.ParameterOwners and take a SynPat as the name identifier.
In practise, this can be SynPat.LongIdent or SynPat.Ident.

Later this could be extended to include SynPat.Operator and SynPat.DotGetOperator.
And after that, the captured SynOperatorName could also be introduced into SynExpr.

@dsyme, @vzarytovskii please let me know your thoughts on this.
I do believe this proposed route should be favoured over the alternative in #13023.
Replacing Ident with SynIdentOrOperatorName takes a performance hit and it is used in a lot of places where operator names will never be valid. So, I would abandon that idea altogether.

//cc @auduchinok

[nojaf]

Closing in favour of #13057.