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

[RikkiGibson]

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

from roslyn.

[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.