Use the Microsoft.Bcl.Memory package for the System.Index/System.Range implementation downlevel

[jkoritzinsky]

The .NET Libraries team has recently shipped a new assembly, Microsoft.Bcl.Memory, which defines the System.Index and System.Range types for downlevel usage. Instead of having an internal System.Index and System.Range type for downlevel usage, it would be nice to use this package as it would also allow analyzers, code fixes, and source generators to use index and range-based language features by referencing (and not shipping) the Microsoft.Bcl.Memory package version that Roslyn provides (similar experience as System.Memory and System.Collections.Immutable).

This would provide a solution long-term for dotnet/runtime#104308, where we have to manually do an internal polyfill of the Index and Range types as shipping Microsoft.Bcl.Memory along with our analyzers/source-generators is extremely difficult.

[sharwell]

as it would also allow analyzers, code fixes, and source generators to use index and range-based language features

This is a difficult claim to make, as each of these compiler extensibility features builds against netstandard2.0 and building with C# language version 8.0 against netstandard2.0 is not a supported scenario.

[jaredpar]

Indeed. These are explicitly not supported scenarios.