This brings support for the final syntax of "ref readonly" and some bug fixes. (#2121)
This brings support for the final syntax of "ref readonly" and some bug fixes. (#2121)

Noticeable changes:
- use `in` at declaration of "in" parameters, not `ref readonly`
- `ref` comes before `partial` when declaring partial ref structs.
- unboxing conversions for ref-like types are statically rejected as impossible to succeed

The corresponding VSIX to match these compiler bits with IDE experience is -
https://dotnet.myget.org/F/roslyn/vsix/0b48e25b-9903-4d8b-ad39-d4cca196e3c7-2.6.0.6221102.vsix
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