Continuation of reference assembly work, initially done in #12334

Generate deterministic MVID for reference assemblies based on contents of the assembly where the content also includes signature and optimization data; basically SHA1 of the bytes of the entire assembly.

Note
We need to keep in mind, that we store some extra information in sigdata, which can affect MVID generation:

• the metadata for "private" declarations in implementation files that don't have signature files
• the metadata for "internal" declarations when there are no InternalsVisibleTo in the assembly

Related issue: #12746