Adding Support for FieldDescSlot generic dictionary entries for R2R generic code...

Adding Support for FieldDescSlot generic dictionary entries for R2R generic code. (dotnet/coreclr#6200)

This enables generic code with ldtoken instructions for fields on generic types to be compiled into R2R.

This change makes newer versions of the runtime compatible with older version R2R images,
but not vice-versa. Therefore, the major version is increased to 2 with this change.

This change adds more encodings to a R2R image without changing the format of any previously encoded
entity, and is backwards compatible.

Commit migrated from https://github.com/dotnet/coreclr/commit/693fff99049bc7d63459b2c444237ca532036b47