Optimize single spread of IEnumerable to array

[RikkiGibson]

Closes #71296
Closes #71755

Some of these issues also discuss optimization in cases when multiple spreads of unknown length are present in the collection-expr, such as [..e1, e2, ..e3] and so on. I am thinking we should just handle the single spread case [..e] for now as this is the source of majority of complaints. This is now handled by simply calling System.Linq.Enumerable.ToArray<T>(IEnumerable<T>) in most cases when the spread value is convertible to IEnumerable.

[RikkiGibson]

There's now 3 possible optimizations this method will try, in order:

1. List.ToArray if the spread value is a list
2. IEnumerable.ToArray if our checks say we should convert the spread value in this way
3. ReadOnlySpan.ToArray if we can convert the spread value to ReadOnlySpan

In my opinion this is the best order to try these, in part because IEnumerable.ToArray tends to produce less IL than the span conversions.

[AlekseyTs]

Consider capturing this information in a comment

[RikkiGibson]

Done

[cston]

object[]

Let's also test when the target type is Span<object>.

[cston]

Perhaps test the following cases:

string[] x = ["a"];
M1(x);
M2(x);
static Span<object> M1(string[] arr) => [..arr];
static Span<object> M2(object[] arr) => [..arr];

[cston]

C.M2(["b"]) creates an object[] rather than a string[]. Consider rewriting as:

string[] s = ["a"];
C.M1(s);
C.M2(s);

[AlekseyTs]

@RikkiGibson It looks like #71755 is going to be partially addressed, but this PR is closing it. Do we have a different issue tracking the work for a more general case?

[AlekseyTs]

Done with review pass (commit 9)

[RikkiGibson]

@RikkiGibson It looks like #71755 is going to be partially addressed, but this PR is closing it. Do we have a different issue tracking the work for a more general case?

Yes, I think #75863 represents the extent of the further improvement we could realistically make to the general case of #71755.

[AlekseyTs]

LGTM (commit 10)