Type providers allocate large unshared assembly metadata that ends up on LOH

[davkean]

Even after we cache creation of type providers in #5929, in the trace they will be allocating up to ~500KB of assembly metadata each per creation, most of which ends up on the LOH heap. This assembly metadata appears to be the same assemblies that are used by not only type providers themselves, but also the compiler. We should attempt to share this metadata to avoid it being loaded over and over again. Ideally, these should be going to through VS's IVsSmartOpenScope service while inside of VS to avoid duplicating metadata across the solution.