Source generator server cache should consider MetadataReferences

[chsienki]

Today, the generator driver compares metadata references via instance. In the compiler server cache, we create a new compilation between invocations; because the references are on the compilation, we don't consider them to be equal, even when the underlying assembly is the same.

In #57177 we added the ability to provide comparers for situations like this. We should add a metadata reference comparer, so that when the underlying assembly is the same we consider it to be equal.

This is especially important for perf in generators like the Razor SG which does a lot of work on the metadata references.

[jaredpar]

Can this be as simple as comparing the MVID between the references?