A few changes required to enable the use of the native PDB reader when debugging minidumps:
1. Consume PDBs even if the module binary is not available
Implementing a PlaceholderObjectFile to complement the existing PlaceholderModule
2. Use the minidump exception record if present
3. Relax the PDB file search
As noted in the comments this is a stop-gap until we add a proper SymbolVendor
Overall this is a modest step towards improving the minidump debugging experience. But more importantly,
it enables the basic consumption of PDBs using the NativePDB(TM) reader as a foundation to add more functionality incrementally.