Add more rationale as to how this exception is different from others.
Callback to external storage now happens explicitly before Sema instantiates.
Remove PCM utility headers left for debugging.
Duplicate declarations of template default argument from rootmap files are just fine.
Annotate template parameters with default arguments as coming from rootmap.

This prevents Sema from complaining about duplicate default argumant declarations.
Add patch for not checking duplicate template default args from rootmap.