Check CS0103 when try to replace type cast by 'is' pattern

[Cosifne]

Fix #51340
In code sample

using System.Linq;
class Bar
{
    private void Foo()
    {
        var objects = new SpecificThingType[100];
        var d = from obj in objects
                let aGenericThing = obj.Prop
                where aGenericTh[||]ing is SpecificThingType
                let specificThing = (SpecificThingType)aGenericThing
                select (obj, specificThing);
    }
}
class SpecificThingType
{
    public SpecificThingType Prop { get; }
}

When we search the potential match for aGenericThing, let specificThing = (SpecificThingType)aGenericThing is included.
However, when the analyzer is trying to apply aGenericThing is SpecificThingType Baz in the WhereClause
it is unsafe to replace (SpecificThingType)aGenericThing with Baz.

So add an extra diagnostic check for CS0103

[CyrusNajmabadi]

i don't really like this approach. i think we should have just changed the analysis step to filter our matches in linq queries clauses.