Windows PDB: Improve reusal of imports scopes

When emitting Windows PDB the compiler doesn't reuse import scopes as much as it could. It currently deduplicates import scopes between two subsequent methods but not across all methods in the compilation. When emitting Portable PDB the scopes are deduplicated across the entire compilation. We can use the same logic to do so for Windows PDBs as well.

Once fixed enable skipped PDB conversion tests refering to this issue.