Generic dictionary minor performance improvement and simplification for R2R (#5690)

Generic dictionary minor performance improvement and simplification for R2R (#5690)

A set of refactoring changes to slighly improve the performance of generic dictionary access on R2R images, and simplifying the codebase:
    1) Removing dependency on CEEInfo::ComputeRuntimeLookupForSharedGenericTokenStatic (and deleting the API).
    2) Stop parsing the generic type/method signatures when generating the R2R dictionary lookup assembly stub.
    3) Avoid re-encoding the generic type/method signatures in a new in-memory blob using a SigBuilder  (signatures used directly from the R2R image)
    4) Moved the parsing/loading of type/method signatures to Dictionary::PopulateEntry()
    5) Dictionary index and slot number are now encoded in the generated assembly stub instead of the signature (stub takes a pointer to a data blob, which contains the signature, the dictionary index and slot, and the module pointer)