In order to have all inputs loaded & available upfront when starting to merge types, we need to open PDB type servers early in the process.
PDB type servers are created when using MSVC /Zi. In that case, the debug info for a given project is stored in a single PDB type server file. All OBJs in that project will only refer to that PDB through a `TypeServer2Record`.
This also fixes a tiny issue where the PDB "Age" wasn't validated against the OBJ's `TypeServer2Record.Age`. The age is used by MSVC incremental compilation, the PDB GUID doesn't change in that case, but the Age is incremented by MSVC on each incremental build.
This patch is step 2. in "Proposed commit strategy" in D59226