Avoid boxing LINQ on `ImmutableArray<T>`

[Flash0ver]

Not all System.Linq.Enumerable extension methods on IEnumerable<out T> have a more specific extension method overload for ImmutableArray<T>, which causes a boxing allocation.

E.g. Enumerable.GroupBy Method

Use a specific ImmutableArray<T> extension method overload to avoid boxing allocations.

This also (partially) applies to Roslyn struct-based collections, like Microsoft.CodeAnalysis.SyntaxTokenList and Microsoft.CodeAnalysis.SyntaxList<TNode>.

E.g.

foreach (AttributeSyntax attribute in node.AttributeLists.SelectMany(static attributeList => attributeList.Attributes))