Generalize (CollectionBuilderType<T>)[..readOnlySpan] optimization to spread operands which are implicitly convertible to ReadOnlySpan<T>

[RikkiGibson]

I would welcome a follow-up PR which also handles cases where the spread operand is implicitly convertible to ReadOnlySpan<T>.

Originally posted by @RikkiGibson in #73102 (comment)

Affected scenarios would include:

Span<int> span = ..;
MyCollectionBuilderType<T> collection = [..span];

// currently:
MyCollectionBuilderType<T> collection = MyCollectionBuilderType.Create((ReadOnlySpan<T>)span.ToArray());

// could change to:
MyCollectionBuilderType<T> collection = MyCollectionBuilderType.Create((ReadOnlySpan<T>)span);

[DoctorKrolic]

Should this apply only to well-known Span type or any collection type with user-defined conversion to ROS, either implicit or explicit? The latter seems pretty reasonable, but I'm not sure whether the spec allows using such arbitrary user-defined members in codegen

[RikkiGibson]

I wouldn't use explicit conversions.

I feel like we should use a user defined implicit conversion to ROS no matter which type defines it. It's possible language needs to extend the definition of "well-behavedness" for us to be able to do that. At minimum this should work with spans and arrays.