Inconsistent inlining between Span.ToArray and ReadOnlySpan.ToArray

[MihaZupan]

It's odd that Span.ToArray is marked AggressiveInlining but ReadOnlySpan.ToArray isn't.
This is the only shared helper where they differ in this way.

Seems to have snuck in during dotnet/corefx#26667

runtime/src/libraries/System.Private.CoreLib/src/System/Span.cs

Lines 435 to 437 in c1b153a

	/// </summary>
	[MethodImpl(MethodImplOptions.AggressiveInlining)]
	public T[] ToArray()

runtime/src/libraries/System.Private.CoreLib/src/System/ReadOnlySpan.cs

Lines 406 to 407 in c1b153a

	/// </summary>
	public T[] ToArray()

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[xtqqczze]

Inlining seems beneficial if the span length is known to be non-zero, as the static ctor for Array.Empty<T>() can then be elided.

[xtqqczze]

Some nice code size improvements by removing AggressiveInlining and splitting the implementation across two methods: MihuBot/runtime-utils#1497